Power and area efficient. You can fit 4 E cores in the area of 1 P core. Adding AVX-512 to the E cores would significantly hamper that, though I don't know by how much.
I've been thinking about this. I think the idea of heterogeneous extensions is nuts. Intel is right to either have it on all cores or none. I think the die space would be worth it to actually have more high performance, efficient vector extensions available. Considering how many E cores are being crammed into raptor lake, I hope Intel will decide to add AVX-512 into E cores in time.