The Anomaly
After the Matriarch Leah gave birth to four sons, she experienced a child-bearing hiatus.  Jacob then married Bilhah and Zilpah who each bore him two sons. Leah’s prayers for another child were answered by G‑d and she gave birth to Yisachar and Zevulun.
Nachmanides notes that there is something anomalous about the way the Torah describes their birth. It says, “G‑d hearkened to Leah; and she conceived and bore Jacob a fifth son.” Similarly, with respect to Zevulun the Torah states: “Then Leah conceived again and bore a sixth son to Jacob.”
When the Torah describes her sons Reuben and Shimon it does not say that she bore Jacob a first and then a second son. Rather it says of Shimon that, “She gave birth to another child.” The same pattern is followed with the birth of Levi and Yehudah. The Torah does not mention that they were her third and fourth children, respectively.
Yet, when she bears Yisachar and Zevulun the Torah makes a point of stressing that they were her fifth and sixth sons, respectively.
Question Magnified
The question is magnified when we consider Rashi’s comment concerning the special gratitude to G‑d she expressed for the birth of her fourth son, Yehudah. Leah was a prophetess, and she knew that Jacob would father 12 tribes, which meant, ostensibly, that each of his four wives would bear him three sons. When she had her fourth son, Yehudah, she was filled with special gratitude because she realized that she had been rewarded with more than her fair share of sons.
This was important to her for many reasons, the least of which was that she felt that her continued ability to bear sons to Jacob would bring him closer to her, making up for the fact that she was the wife that he did not choose and who was forced upon him by his wily father-in-law, Laban.
Yet, despite the significance of Leah bearing a fourth son to Jacob, the Torah did not find it necessary to stress that Yehudah was her fourth child and thus one more than her “fair share.” Why then, does the Torah emphasize that Yisachar was her fifth and Zevulun her sixth child?
Connection to Torah Study
Of all the twelve tribes, Yisachar and Zevulun stood out because of their unique relationship with Torah study. The tribe of Yisachar was known for its mastery of the very complex laws concerning the sanctification of the new moon, which required extensive knowledge of the intricacies of astronomy. Indeed, Maimonides writes that they actually composed astronomical works prior to the Greeks which were eventually lost. The Yisacharites were totally devoted to Torah study.
Zevulun’s tribe made their living as merchants.  They made a pact with the members of the tribe of Yisachar to support them so that they might share in the merit of their Torah study.
To highlight the relationship of these two tribes to Torah study, the Torah indicates that they were the fifth and sixth sons of Jacob because the numbers five and six allude to and represent dedication to the study of Torah. The Five books of Moses that form the foundation of Torah knowledge were given at Mount Sinai on the sixth day of the month of Sivan (See, Alshich). Alternatively, there are Six Orders of the Mishnah and Talmud which embody the written version of the Oral Law first given at Sinai.
According to some commentators, Yisachar was conceived on the night of Shavuos, the sixth day of Sivan, the very day on which, years later, G‑d gave us the Torah at Mount Sinai. Yisachar’s organic connection to the Torah was established and instilled in him from birth. And since Zevulun made Yisachar’s dedication to Torah study possible, he too was considered to be inseparable from Torah.
What about Levi and Yehudah?
One can raise a simple question here. Granted that Yisachar was distinguished because of his Torah knowledge, but so were the tribes of Levi and Yehudah. The tribe of Levi produced the Kohanim and Levi’im, who devoted themselves to spiritual pursuits such as Torah study. Likewise the tribe of Yehudah produced many great Torah scholars who served in the Sanhedrin. Why then were Yisachar and Zevulun the ones singled out to be identified with the “five” and the “six” of Torah?
The answer lies in the unique arrangement between these two tribes as mentioned above. Zevulun undertook to support Yisachar, and as a result the tribe of Yisachar was able to devote all of its time and energy to Torah study. The Yisacharites had no other concerns than Torah.
Although Torah study must be augmented with good deeds, such as reaching out to help others, there are some select individuals whose mission in life is to devote most of their time and effort to Torah study.  
Unadulterated Torah Study
At the end of the Book of Esther we are told that Mordechai, the hero of Purim, was “accepted by most of his brethren.” Why not all? The Talmud (Megillah 16b) answers that when Mordechai was appointed by Achashverosh to be his Prime Minister, it took away time from his Torah study. Granted, the majority of his colleagues understood that the exigencies of the moment demanded that he take off some time to lead the Jewish people. Nevertheless, because of his political duties he was no longer able to join in with his brethren who had the “luxury” to engage in uninterrupted Torah study (See Likkutei Sichos volume 16, p. 373 for an extensive treatment of this matter).
Indeed, the Talmud (ibid.) states that Ezra, the leader of the Jewish people at the beginning of the Second Temple Era, did not join his brethren to build the Bais Hamikdash as long as his Torah teacher, Baruch ben Neriah remained alive. From this, the Talmud derives that unmitigated Torah study takes precedence even over the building of the Bais Hamikdash! Obviously, Ezra was the exception to the rule. As long as there was another able leader to be involved in leading the people, Ezra was able to devote himself to exclusive Torah study.
Moshiach is referred to as both a King and a teacher. The Rebbe explains (Likkutei Sichos, volume 35, p. 210) that his role as King will be superseded by his role as teacher because after he builds the Bais Hamikdash, gathers all of the Jews to Israel and achieves all the goals of a peaceful, united, moral, stable and spiritual society, he will no longer need to exercise his royal status; he will be able to devote himself exclusively to Torah teaching, for that, indeed, is the ultimate goal of the Messianic Age.
In the words of Maimonides at the very end of his Mishneh Torah:
“The entire world’s preoccupation will be to know G‑d exclusively.” “Knowing” G‑d is an integral part of the inner dimension of Torah study.
The provenance of the Messianic Age’s total and exclusive dedication to Torah study was the birth of the tribes of Yisachar and Zevulun, the two tribes that personified the ideals of  total devotion to Torah study, as hinted by their designation as the ”fifth” and “sixth” sons of Jacob.
Chapter Eleven
One may add that their connection to the Messianic Torah dynamic is hinted in the numbers five and six. When added together (since Yisachar and Zevulun were essentially inseparable) they yield the number 11, which is the number associated with Moshiach, whose spiritual level transcends the conventional system of Ten Divine Attributes.
It is no coincidence that the most detailed description of in the Torah of Moshiach occurs in the eleventh chapter of Isaiah. Likewise the most comprehensive treatment of Moshiach in the Talmud is in the eleventh chapter of tractate Sanhedrin. Furthermore, Maimonides devoted the eleventh chapter of the Laws of Kings, in his Mishneh Torah, to the subject of Moshiach.
It could be said that the Messianic Age will bankrupt (by filing a “Chapter Eleven”) the negative energies of Galus that necessitate the daily disruption of our pursuit of Torah knowledge!