Beyond the security issues, Adobe was just terrible as a platform maintained. Flash was the largest source of crashes in every browser until they moved it to a separate process, there were endemic performance issues even in core features (e.g. video playback was significantly more CPU intensive, and general code was well behind native speed), and most of the toolchain was in a state of obvious neglect, from the library inconsistencies (e.g. fonts rendered differently in some controls in the standard library and many of the method/property names didn't match), to the wrong or missing documentation, the many crashing bugs in the IDE, and the debugger which couldn't even pause on a breakpoint reliably. Nothing ever got fixed because their resources went into new features but the support people would reliably ignore tickets until sending a message telling you that it might be fixed in the next paid upgrade - could you buy it and let them know?

Had Adobe shown even the slightest sign of responsibility it might have been a different call but at any point after maybe 2002 it was pretty obvious that you shouldn't depend on Flash because Adobe was just milking the customers.