There are perfectly functional ARM implementation of flash out there. ARM is the low power RISC chip on lots of mobile phones.
Why its not on the iPhone is a mystery.
I'd suggest it's because Adobe's optimisation of software has been a real problem of late and Apple's keen not to let the shitey performance/lack of responsiveness of Flash become an issue on the iphone. Particularly given that Flash tends to work in a more proprietary way than Apple would like for consistency.
Adobe came out last week or so and claimed that they've now got Flash running 3x as fast as the present lamentable version on the mac, apparently because 'they have 'identified the bottleneck in the Flash Player for OSX. Like in the other plugins the culprit is text rendering, in this case rendering using device text.," . Which begs the question how the fuck did it take so long to reach that conclusion
Since Adobe became so dominant they've shown the same slapdash attitude that characterised Quark in its declining days. The prices are up and the software's not as sharp as it should be