The major thing is that the FAA loves to approve what someone at the FAA has already previously approved. So if you have parts of the aircraft, avionics, pilot

The major thing is that the FAA loves to approve what someone at the FAA has already previously approved. So if you have parts of the aircraft, avionics, pilot operating manual, performance values, etc that have previously been approved by the FAA then it is much easier to get them approved again because the precedent exists that it is ok. Someone else did all the heavy-lifting to get that first approval signature. By classifying them as the relatively rare "Powered-lift" category they have created a huge bureaucratic hurdle to overcome. You've gone from thousands of approved airframes to a handful of approved airframes. You can still work through it but there isn't a precedent for them to easily point to for fast approval.