To me this is just a simple artifact of size & attention.

Another example of this is stuff like Bluesky. There's a lot of reasons to hate Twitter/X, but people going "Wow, Bluesky is so amazing, there's no ads and it's so much less toxic!" aren't complimenting Bluesky, they're just noting that it's smaller, has less attention, and so they don't have ads or the toxic masses YET.

GenAI image generation is an obvious vector for all sorts of problems, from copyrighted material, to real life people, to porn, and so on. OpenAI and Google have to be extraordinarily strict about this due to all the attention on them, and so end up locking down artistic expression dramatically.

Midjourney and Stable Diffision may have equal stature amongst tech people, but in the public sphere they're unknowns. So they can get away with more risk.