For the vast majority of social games, which are written in Adobe's proprietary Flash environment, getting up and running on a smarthpone like the iPhone or Android is impossible or, at best, a spotty proposition. This means that a Flash game developer has to take the time and effort to create special versions of their games to run on each type of smartphone, as Zynga recently did with its Farmville iPhone app. While this might be doable for the big boys, smaller developers simply don't have the resources to devote to this kind of targeted development.
Games created with the open standards of HTML5, on the other hand, can be easily run on most smartphones today, without any need for specially created versions. If HTML5 development becomes a trend in the social gaming world, that bright line between social gaming on the phone and on the PC may disappear entirely.
Of course there are still some issues with game development HTML5 (see here for a technical explanation), but they will likely start to recede as the technology becomes more popular and robust. Kudos to Heyzap for helping that process along.