Whether or not he especially wants to end "birthright citizenship" (he may), or intends to try to do so in an unconstitutional manner (he may), the answer to "why now?" is:
Because he hopes that putting the proposal into the news cycle right now will get more of his "base" voters off their asses and to the polls for the midterm election.
Yes, it's really that simple, and no, there's not really any more to it than that. There is a secondary question related to "why now?" that proves it. That question is "why not at any other time in the past nearly two years, when he could have actually taken action instead of just flapping his gums?"