That's because it's always all about combining liberal 'feel good' virtue-signaling with the true leftist goal of chipping away at the second amendment with mini-laws that progressively restrict the freedom to own weapons one little law at a time.
They have decided that it's easier to start with the AR-15. That's really the only reason. Once they manage to outlaw those . . . and then rifles in general it will be much easier to go back, tweak those laws a little bit in their 'interpretation' and then go after handguns as well.
Total disarming of the population is the true goal . . . and the creation of a nation of serfs that have got absolutely no choice to depend upon the government for their freedoms . . . if any.