It's easier to put a small thing into a big space rather than a big thing into a small space.
Physically a P90 is a similar size to a humbucker so not so difficult to do.
Squishing a HB down to a Fender single coil size comes with a lot of comprimise.
If you don't like single coil pickups - play Humbuckers
Actually Splitting some of the humbuckers and using coils together like on the Petrucci guitars works well
I can see the logic in that, but some would say that the MQ is a compromise too, and it's not everyone that sees single size hums as a compromise because they do have a sound of their own. Ask Andy Timmons.
I agree with the last part. But I figure that's the reason that on a Strat the 2 and 4 positions are the only I like. On an HSH configuration, I would use a true singlecoil in the middle, but only use it with the split humbuckers.