I'm always hovering around waiting for someone to ring.
There are no real technical limitations preventing it from being done. However, there are some practical reasons not to. Firstly, not all browsers support HTML5 (earlier versions of IE come to mind), so moving it might actually make it less accessible. Secondly, it is much harder (in my experience) to write and develop a game in HTML5/Javascript than in Flash/Actionscript, at least for now. Most importantly, Kongregate, which is an important source of traffic and revenue for the game, only supports Flash games for now, so moving it now is really out of the question.