Surely, if Yaakov would have indeed revealed the time of Moshiach's coming to his children [Vayechi 49:1], they would have been totally devastated to hear that they had so long to wait.
When Yaakov's sons would hear that Moshiach was not scheduled to come for a long time, they would have realized that some considerable additional effort was needed to bring him sooner - as the Talmud states that through additional merit the Redemption comes earlier [Sandhedrin 98a]. Thus, Yaakov hoped that by revealing that ''the End of Days'' was a long way off, it would motivate his children to add substantially in Divine Service, so as to bring Moshiach sooner.
Nevertheless, despite his good intentions ''the Shechinah departed from him'' and Yaakov found himself unable to reveal the ''End of Days''. For, ultimately, God wants us to bring Moshiach through our own efforts, and not through the assistance of ''revelations'' from Above.
Based on Likutei Sichos vol 20 Lubavitcher Rebbe