HTML5 is a pretty iffy target platform for games today. It is very difficult to support all the different browsers across desktop and mobile well and still have fast audio and even just simple 2D graphics. That's today.. in 2007 (when Zynga was started) HTML5 basically didn't exist as anything other than a pipe dream. If Zynga had "bet on HTML5" back in 2007 they would have been a stillborn company that never made any profit to begin with. For the types of games they make, Flash was inarguably the only option then and even today on desktop browsers it is arguably still the only reasonable option.
Yes, HTML5 will be great for games, maybe in 2014 or so. I'm sure they have people there working with HTML5 and getting ready for that.
It would be nice to get automatic compatibility with anything that runs a modern web browser, but the sad reality is that most PCs don't use a modern web browser, and HTML 5 doesn't really simplify anything. Flash runs the same everywhere from only one codebase.
http://techcrunch.com/2010/09/24/zynga-continues-internation...