Flash grew up in a very weird time where nobody cared about hardware acceleration at all. Go back just a few years and even basic multimedia tasks like playing music or decoding a JPEG require some amount of accelerator support. Go forward a few years and hardware accelerators are suddenly crucial to energy efficiency. Ergo, everything in Flash Player is done in software. Vector rendering & compositing? Software. Video decoding & playback? Software. This wasn't at all exclusive to Flash, of course, but Adobe was uniquely unsuited to realize this problem.

Apple realized very early on that you needed hardware acceleration for common computing tasks in order to have laptops that last more than an hour on a single charge. It took Adobe several years after the launch of the iPhone to actually support some amount of hardware accelerated video decode, and as far as I'm aware vector rendering (the core of the platform) was never meaningfully accelerated. As far as I'm aware, pre-iPhone efforts to get onto phones involved a special cut-down Flash runtime that was perpetually out-of-date and was only really popular on early-2000s Japanese handsets. This wasn't at all built for the "computer in your pocket" that Jobs wanted.