The first Chabad Rebbe, Rabbi Shneur Zalman of Liadi explained the reason for the lengthy exile:[1]
The Talmud (Yuma 9b) that the destruction of the second Temple was on account of the sin of baseless hatred towards ones fellow.
However, people do not consider this sin to be serious as they delude themselves into thinking that their hate is justified.
Therefore, this exile whose purpose is to rectify this iniquity, requires much time. For, to remedy a hidden evil is much more difficult and requires more time than to rectify something that is clearly evil.
The lesson is clear: When we cease convincing ourselves that we have a right to hate our fellow and instead show our fellow baseless love, the redemption will immediately arrive.
[1] Likutei Torah, Matos 86a
