Because people haven't caught on yet. Browsers will start to support it and it will catch on once people realize its vast benefits over PNG. For now, it's not worth it for people to switch.
It may be tough, but porting your application to Windows would be a great idea and help you compete against Google in the long run. Good luck with it, and from what I've checked around on your website it seems like a great tool/competitor to Google!
(I'm on a Mac, still I have Windows machines and know a huge amount of web-developers who abide by Windows)