A big reason is that the RNC is totally centralized and removed from the state and local parties and organizations. So they have little access to the footsoldiers who do this stuff. Having been to many local office debates in NYC and Nassau County I can testify that at the Q&As 90% of the Qs are asked by plants from BOTH parties. It is a time honored practice, though it does often exclude the actual voter. The closest I have seen the GOP do this were the townhalls on Obamacare that were attended by the nascent Tea Party. But then the Tea Party idea was overtaken by politicians who sought to ride its coattails topower and then forget it.
What’s to stop us from doing the same? Plant someone at a Dem rally and ask if Clinton believes that the Benghazi parents are liars.