Not having tried it, my first guess would be the ongoing DRM problems the 21 has had. However, there could just be such a large number of serious bugs they feel it easier to tell people not to bother to use it in 2 and stick to 1.5 pending the next patch.
The Jedi Master
The anteater is wearing the bagel because he's a reindeer princess. -- my 4 yr old daughter