cjelly is half-correct...The sets are Lego store exclusive (that I've seen), but you can order them off of Amazon or the Lego.com shopping page.
As with all OOP sets, there's never really been a discussion as to why Lego hasn't brought certain ones back. I mean, look at Taj Mahal. That set fetches upwards of $2000 on eBay/Amazon/Bricklink. Retail was what, in the two-hundred dollar range? People who purchase these sets on the after market are not giving a dime to Lego, and as a result, Lego's decision to not reprint the set is [indirectly] taking profit from them.
The way I see it, if Lego did a limited run of some of these crazy high dollar sets, they'd make an unfair amount of money.
Unfair amount of money.