Minor reasons: less performant than Javascript solutions (the transition is sluggish and jittery in Chrome for me) and the fact that I have to click a little target to shrink it back, whereas most Javascript solutions will let me hit Escape or click elsewhere on the page to shrink it back.

Major reason: Separation of concerns: CSS is for appearance, Javascript is for behavior. They don't work well trying to do the other's job. This is touched on by the shrinking problem I mentioned above - it's easy to watch more events to make the behavior of Javascript functionality more flexible or more user-friendly, but not so much in CSS.

That said, it's a toy demonstration of transitions; it's cool that the bear dances, even if I wouldn't take him on Dancing with the Stars.