DRM is not the reason youtube uses flash, browser support and feature usage. HTML5 video APIs are still frustrating to use compared to flash's pretty well built-out mechanisms.
Advertisers also love to serve video ads on a flash platform because it lets them perform custom analytics, logic, etc.
And don't forget the part where flash is a backdoor to users computers. It can bypass proxies and give them your real IP, and I'm not sure even google will want that to go away.