Wonder why they don't place them straight away into the memorial Chapel, is it traditional that they do time in the vault first, can people visit them in the memorial Chapel or vault. Fascinates me.
Probably because of the work involved.
The vault can serve as final resting place as well, there's pics of it and it's like bunk beds with coffins behind iron gates and plates noting who it is.
Some kings are permanently there (for now?) as well as eg q Vic's father.
For Philip it's a temporary parking place and if Liz had gone before him, I think she'd be temporarily there as well, waiting for him.
Margaret's ashes were there temporarily, too.
I think the chapel was purpose built for Liz qm & George + Liz & Phil (in the ca 1960s, G was also in the vault before being transferred and buried in the chapel).
It is the only new addition to the castle (or just chapel?) since the 1500s and Margaret had to be cremated (highly unusual for them), because she wanted to be buried with her father but there was no space. She was interred in the vault and then buried at the same time as Liz qm.
(I suppose everyone expected Marg would be buried at Frog, possibly with a husband, but her life turnt out differently)
Which, to answer your question, makes me think it's a logistical decision, as it means building work (opening the designated place(s), setting in the coffin, closing again) is easier to do once for two people than twice (esp if the span between the two is short).