It's probably because they haven't paid attention to software rendering. Many people don't seem to appreciate how well most popular UI toolkits work if your GPU

It's probably because they haven't paid attention to software rendering. Many people don't seem to appreciate how well most popular UI toolkits work if your GPU drivers are incomplete or missing. (or if you just don't have hardware that supports modern OpenGL APIs.) This, among other more Googly reasons, is why I'm personally not a flutter fan. It's not like there aren't other decent cross platform UI toolkits anyway.