the issue is not the availability of graphics libraries or the competence of developers, but the fact that Flash is proprietary software and a proprietary format, with little to no documentation available (other than how to create Flash movies or apps).

developers have to tediously reverse-engineer Flash's features, and that takes time and is error-prone. people with no background in programming tend to oversimplify this, but it really is hard work.
take a look at gnash, for example. this project exists for a number of years, and still has a long way to go.

and, to be honest - trying to provide an open source implementation of a proprietary format isn't exactly something OSS developers like to do, because it benefits the company who doesn't want to share its code in the first place.
i'd rather see flash die and alternatives like HTML5 or SVG take over instead of further leveraging Adobe on other platforms.