DE Rebecca..Can we not unvault Frost and Ember please. Nyx, Loki and Volt instead please.

[Loswaith]

I'd expect its based more on habit (or not a re-look at the system) as the void key system necessitated the need to change around and vault primes, which trickled over to the whole unvaulting process as players asked for access to the earlier primes. 

The relic system changed that quite a bit as it now teaches players a different method (you can wait) than the void keys did (which was more along the lines of get it while you can, which was closer to the more traditional limited offer style ).