Why was Wegener's idea of ""continental displacement"" not widely accepted at first? What did it lack?

According to Wegener, all the continents were all one lumped together into one large land mass before they started drifting apart over time over the oceanic bed.

At first, the idea was highly criticized because he never explained "how" they drifted apart, as in what caused the motion, what was behind the drifting. I hope you understand.