Uh, no. It's because when plane engines break they fall out of the sky onto things. They break when they aren't correctly modified to take the new fuel. It is not very realistic to mandate conversions for all of the (often quite old) planes today- like from manufacturers going out of business. It is not trivial to convert an engine, fuel system etc. and also meet the same performance so that things like stall and thrust and altitude are still accurate across the important ranges.

In practice, it's a choice between:

1. Banning or buying out older planes at not-insignificant cost 2. Swapping fuels and doing your best to get people to fix their planes, causing crashes 3. Playing it slow, requiring new planes to be compatible with lead-free fuel, building new lead-free infrastructure, and gradually encouraging people to transition

Currently we're doing 3 until it's determined that 1 is cheap enough to do. That said the FAA could certainly be doing more, but that's much more (IMO) on them not taking the issue seriously than on voters being appeased. Its a national agency, not the local school board.