Because it is very hard to find new hardware to test it, let alone expect your users to take advantage of it on their machines.

AVX512 is such a mess that Intel just removed it after a generation or two. And on ARM SVE side it is even worse. There is already SVE2, but good luck finding even a SVE enabled machine.

Apple does not support it on their Apple Silicon™ (only SME), Snapdragon does not support it even on their latest 8 Elite. 8 Elite Gen 2 is supposed to come with it.

Only Mediatek and Neoverse chips support them. So finding one machine to develop and test such code can be a little difficult.