The force was beginning to weaken.
By Darth Bane's time (1000 BBY) Darth Bane remarks that the dark side is diluted between many force users or "Dark Lords" He "Fixes" this by installing the rule of two. One master, one apprentice.
It is likely due to the (often violent) deaths of force users over the centuries, the Force has withdrawing from the galaxy.
Did you even read the Bane Trilogy?
It strictly states, the Dark Side is stronger when it is concentrated on smaller numbers.