Flash did not integrate well with the rest of the page and was an utter nightmare with respect to accessibility and responsive design. It was also buggy, prone to memory leaks and crashing, and required the end-user to install or update arbitrary code on their machine, with all the headache and security implications that necessitates. Support and performance was also horrible on mobile when it was available at all. Flash was pretty and had decent tooling, but it sucked pretty hard compared to today's options.