I can think of two reasons of the top of my head why the PC is the right platform for casual gamers. People just getting into gaming, first of all, don’t even own a console, but they almost certainly own a computer (and let’s be honest, a lot are playing at work). That tips the scales already — your target audience self-selects. Secondly, gateway casual games are ones that utilize interfaces people already know. Pointing and clicking, or basic four-directional movement, things that don’t overwhelm soccer moms. Hence the success of games like Bejeweled and Diner Dash.
Ubisoft’s lead game designer Emeric Thoa appears to think along these lines as well. With the casual market expanding, it’s worth checking out a few baskets before putting all your eggs into one (small game developers don’t have the budget for multi-platform releases) — and the PC at this point is the winner by far.