It was not accepted during Wegener's lifetime because, mainstream scientists that time didn't know the mantle was liquid, they thought it was solid. They thought the throry was illegitimate because continents did not fit exactly together.
The evidence supporting Wegener's theory was all discovered after his lifetime, like the Seismographs revealed a pattern of volcanoes and earthquakes, and Radiometric dating of rocks revealed a surprisingly young oceanic crust.
