Because I typically see GTK as an ancient buggy toolkit with bad documentation and an ugly cross-platform experience. I would love to be proven wrong though.

There was an entire talk given about the pain of needing to switch Linus Torvalds' divelog app from GTK to Qt and they seemed to have the same opinions I do: https://www.youtube.com/watch?v=gGZyVSOnqm0

Perhaps GTK4 is different and looks better cross-platform, I don't know, but I haven't heard of any new/prominent cross-platform apps that actually use it.

And the way I have been observing several top gnome devs' attitudes and abusiveness towards their own users the last handful of years does not sit well with me.