I believe the main reason they disable them - and they don't follow ideas like keeping the difficulty up for old raids or other content - is that when they go to a new major iteration of the game - 7.x soon - the game balance completely changes, and that includes the bosses as well, the numbers the players can do to them, the utilities they can use to them. Sure, some bosses would be no problem but I bet various obscure raiding or 5man mechanics would be imbalanced if they just kept the bosses high level without changing anything.
edit: I forgot to say, to put it simply, it's likely the lack of resources to maintain them at a balanced state. It would not be worth it. Most players would have done them already so they would not even be popular, and they have a current expansion to maintain and bug fix for months, to not say perpetually.