As far as I know, it was mostly because in his last decades he focused his research mostly on obtaining a classical field theory that unified gravity and electromagnetism, hoping that out of it the discrete aspects of quantum theory would emerge organically. Most of the forefront theoretical physicists viewed this (correctly, in retrospect) as a dead end and focused on the new discoveries on nuclear structure and elementary particles, on understanding the structure of quantum field theory, etc.

Einstein's philosophical criticism of quantum theory was not the reason for his relative marginalization, except insofar as it may have influenced his research choices.