20th Anniversary Silver Eagle sets were available for quite some time...

The 20th Anniversary Silver Eagle sets were available for a long time, well until Christmas of 2006. I remember having wondered about that. The 3 coin gold set was sold out the first day. That is what some remember, and attribute to the silver sets.

As for the relative rarity of the 2008 with 07 reverse, I believe it is being overhyped. Yes! the bidding wars on eBay are crazy, but they are mostly driven by a few who want to hoard them en masse. Wait until the dust settles, which may be a couple of years, and then the true values will become apparent.