I'm not sure I get it.
It seems to me that working even when off-line, even when away from my computer, would be top requirements for any calendar product. In other words, a downloadable application (Outlook anyone? iCal? Blackberry?)
I'd be worried if I could only access my calendar on live web connection.
Sure, GMail is hugely popular, and you could make that same argument there. But GMail lets me synch with any POP-client of which there are many. I use GMail with Apple (Mac OS X) Mail when I am at my primary computer, and over the web everywhere else (including my cell phone.)