Why were Chassidic teachings revealed only in recent generations and not earlier?

There are two explanations as to why pnimiyus haTorah, the mystical dimension of the Torah, was revealed precisely in these last generations, as the footsteps of Moshiach are approaching.

Since the darkness of exile is thickening, an ever more brilliant light is required to pierce it. For this reason the “soul of the Torah” was revealed, for this is what awakens and uncovers the “hidden point of the soul,” i.e., those faculties of the soul that are most deeply concealed.

Since we are now at the very end of the exile, we have been given a foretaste of the Torah's secrets which will be fully revealed only in time to come. This foretaste serves as a preparation for the Days of Moshiach, the main point of which is a knowledge of G-d. As Rambam writes, “The occupation of the entire world will be solely to know G-d..., and the Jews...will attain an understanding of their Creator....”

The first of these reasons emphasizes the lowly state of the latter generations, while the second highlights their distinctive privilege – the fact that they have been found worthy of being granted a glimmer of the future revelation. Nevertheless, though they are opposites, these two reasons are interdependent. For the quintessential power of the inner dimension of the Torah, i.e., the Torah teachings of Moshiach, comes to the fore in its ability to light up even the redoubled darkness of the days that anticipate the footsteps of Moshiach.

Likkutei Sichos, Vol. XV, p. 282