Because the U2 is intended to be overclockable out-of-the-box via software control.  Naturally, people will be trying this and, knowing nothing of how heat transfer works, would be running into issues with them overheating with the U1's thin plate heatsink.  I suspect you could still make the U2 overheat, too, anyway.

I figure if you've got 50 in one of those bulky USB hubs, a custom cooling solution is probably not out of reach if the physical dimensions throw a spanner into the operation.  Or just get another hub