Only reason I can come up is lack of space and overstock of 5.0 engines! Between my brother and I there are 5 or 6 already but only three cars they could go in.
I think the P heads altered the spark plug location which made it difficult to find headers to fit it.
Yep, instead of approx 45* angle, plugs in P heads angle only slightly. Usually #5/6 plug and std exhaust manifolds or headers collide. AFAIK the Explorer headers won't fit any other chassis(and are very restrive). I dunno if aftermarket headers for 5.0 Mustang/P heads are still manufactured, won't fit a Mav/Comet anyway.