I'm no game dev, but wouldn't porting from OpenGL to WebGL be much simpler than porting to 2D Canvas? I know IE doesn't support WebGL, so I'm just wondering how much extra work they created for themselves.
Safari doesn't support WebGL either because of security issues (AFAIK only Firefox does) so while it might be easier it wouldn't work.
but I see what you mean about compatibility, with canvas they get ie9 + firefox + chrome + safari, almost everything for desktops.