Plus it wasn't very good on mobile, even when it was supported (ie - on Android it was still bad and never got out of beta anyhow)
When it worked it was... not great, due to UI issues, but it at least worked.
Trouble is it seemed to not consistently work. I worked at a flash game developer at the time (Zynga) and it would not always load our games even if it had done so previously - the difference was sometimes just refreshing a page.
People always complained about how Flash "drained batteries" / "crashed browser" / "allows all this intrusive advertising". A case of blaming the technology for the way it was being used. People would say "I can't wait until Flash dies and we have the open web standards instead so we don't have these problems any more".
So, fast forward a decade or so, Flash has been replaced by open web standards and ... guess what? We still have those problems...