Servitude had been very common in the West, and relatively absent in the East in the High Middle Ages. Later, the situation was reversed.
Strong monarchs don`t necessarily help, see Muscovy`s/Russia`s example.
In the Late Middle Ages, you had economically strong and important towns with skilled craftsmen; they recruited their labour from the surrounding countryside, so abolishing servitude was a means to increase these economic dynamics and profit from them. Also, centralising states in the West began to rely on mass infantry including firearms militarily.
In the East, on the other hand, technological development in the crafts was lagging behind (hence the many German population imports), and cavalry remained dominant for a much longer time (also considering that the enemies you encountered were also primarily light cavalry).
Then, there is
@cerebus ` good point about later grain supplies: Eastern Europe had become fertile and was still thinly populated, it could become a net exporter of agricultural products, given that other options were not likely (the West was ahead in technology).
Therefore, to have no servitude in all of Europe, you`d probably need to keep technological advances evenly distributed in the West and East, and towns more important than the nobility, or at least equally important, in both parts of the continent. No Mongol invasion might be a key moment here.