I suspect the issue is that E cores are important to Alder Lake's multicore FP throughput. They help with Intel's usual problem with P cores, the giant gap between base (all-core) and max turbo (1-core) frequency. That's why the top Alder Lake config is 8P+8E; you don't put 8 efficiency cores in if they're just for low intensity background tasks.This seems like something that could have been handled in software without dropping AVX512 instruction support from the P cores. It can't be that difficult to schedule executables with AVX512 instructions to run on the P cores only.
AVX-intensive software is exactly that kind of throughput compute load, so restricting it to run only on P cores wouldn't be great. It's conceivable that after you account for rolling back to base frequency on the P cores and taking the usual AVX512 frequency haircut, you get more FLOPs out of 8P+8E * 256-bit AVX2 than 8P * 512-bit AVX512.
There's also this: it's been at least 8 years since Intel started talking about AVX512, yet they've botched its rollout so much it's completely impossible to depend on it being available in the average PC. Software vendors haven't been eager to adopt it at all. Intel may now regard it as a HPC/server feature rather than a client feature.