Why aren't there two vaults?

I assume someone has asked this before, but I haven't seen an answer anywhere yet.

Why aren't the two vault bosses in seperate instances? They require a totally different gear//skill level to down, they have no business being in the same instance.
There's a large group of people who want to do the old boss, and are perfectly capable of doing the old boss, but simpily don't have the skill//gear to do the new one. At the same time there's plenty of people who can easily do the new boss, but have no reason to do the old one because they have all the gear off him already.
These bosses are two different tiers of progression, linking them to the same instance makes about as much sense as having naxx and uldaur linked.