The 50D and T2i are no longer in production, so it's only natural for those to be in the program. That being said, I'm willing to bet that the 50D will be out of the program this year (along with the last of the non Txi Rebels, if they aren't already out) simply because they're getting harder and harder to come by.
Also, since the 60D and T3i are the latest generation of the consumer and prosumer level cameras; there are more of them out and about, which means more will come back for repair/refurbishment/etc, i.e. Canon wants to move that stock. In reality, Canon could hypothetically be making more money on refurbs than they do on BNIB bodies because they've already been paid for the body being sold once. Those went on the CLP about 1 year after release, because of the product life of these cameras themselves being shorter than that of, let's say, the 7D or 5D2.
The drop of the 5D2 from $2500 to $2200 is likely in anticipation of the beginning of the end of its production. When production officially stops, Canon may reduce the MSRP one last time, but don't expect Canon to give them away. These units still cost a pretty penny to be produced, and that cost doesn't magically evaporate once they stop production - it's already been paid for the units in inventory that are left to be sold. The refurbished price being adjusted only made sense because the refurb sales would suffer if they were only a ~10% discount on the new MSRP.
Another thing to consider is that the CLP isn't just an outlet for people to save 20% on refurbished gear; the aim of the program was to be used in place of fixing old, broken gear (hence why you send Canon an out of warranty Canon camera - it's not just for recycling, it's to satisfy the requirement that you tried to get something fixed under warranty and were unable to do so, so instead of repairing the camera you send in, you replace your camera with a camera in the program at a discount).
In honesty... as strange as it seems, adding the 5D2 to the CLP inherently drops the value of the 5D2. The average mean price of a used 5D2 will drop from it's not too recent $1900 range down to the current low end, $1400-$1500. That doesn't bode well for people that just picked up their bodies; by retaining inherent value, people feel invested in Canon. Beyond ergonomics, features, etc, when people think 'Well, I have a $2500 camera, I need to put good glass on it', it moves product for them. People feel more secure about their investment. If the inherent used value of the 5D2 dropped overnight to $1500, overly analytical people could easily start second guessing their decisions to stay with Canon.
It's a stretch, I realize this, but as much as we'd love to see the 5D2 get added back to the program, I'm OK with it not - at least for the immediate, foreseeable future.