[mips_r4300i]

Not an expert at all, but there is a complexity wall when you get to newer Flash with ActionScript (3)?

My very rough guess is that post late-2000s content frequently needs these features and it's at least an order of magnitude work to recreate them

[extraduder_ire]

I think it's more down to engrained quirks in the existing implementation. IIRC, adobe gave mozilla an open-source implementation of actionscript 3 back in the day to try and help legitimize it as an ECMAscript implementation.