There are advantages to live bearing in terms of the ultimate success of young, and thus of the parents in reproducing; these are probably hard to change so that natural selection reverses the course of evolution. Also, there are many complex structures needed for live-bearing, and it would be rare to reverse the appearance of these--many back mutations would be needed at once to return to successful egg-laying.
Work Step by Step
See here that there are complex structures requiring a long evolutionary history in the case of live bearing. Further, think about the evolutionary/reproductive advantages of live bearing.