Flash had a VM that truly was "write once, run on anything" and a great (software) pixel and vector renderer. Cross-browser compatibility with HTML standards was completely bonkers and HTML had none of the features Flash Games and Animations needed. Try to move or rotate a picture at 30 fps with old HTML.
I remember Flash being slow and buggy on Mac and Linux at the time, and updates lagged far behind the Windows version (on Linux in particular). And then I saw an article from a Macromedia/Adobe dev saying that it was heavily optimized x86 assembly code targeted towards Windows and very difficult to port to other platforms... given the IE6 monoculture at the time, it made sense, but I felt like a redheaded stepchild for daring to use a minority OS.
I don't think it was 64-bit clean, either, so now no modern device runs Flash "natively."
