Like I said above, to me, this represents they have re-evaluated where Metroid sits in their property portfolio and realised it is the sort of property they need to invest in seriously, rather than releasing subpar titles.
The other point to factor in, is that IF and JUST IF, Retro had been working on Metroid Prime 1-3 Remasters/Remixes/Remakes (ala Capcoms RE remakes), then potentially another reason they've shifted it from original studio to Retro, is they've been more impressed with the work they've seen from Retro, and figured they already have assets that can transfer between the projects, so have brought across the producer to benefit all these projects at once.
Why make two HD, high quality models of Samus, when you can use one...for example. Ok that's an oversimplification but you know what I mean; there are certainly visual and sound assets that could be shared between Metroid projects, and reduce the need for duplication, given shared engine and target platform.
Consolidate all your development for the property into one location, so resources, attention and quality isn't split, and you have consistency.
Similar to how GameFreak essentially used Lets Go to figure out the Switch and to get newer developers up to scratch, to theoretically have a better handle on the hardware come the next-gen titles.