This was a project by the original developers, not by gearbox. The original game, while incredible, had stability, optimization, and probably most importantly, multiplayer issues.
The community had asked for a way to play together online without third party solutions, but the code base needed to be rebuilt to facilitate that.
We didn't. The new publisher (Gearbox) apparently did.
EDIT: Apparently it was already started by the original devs even before the acquisition, see the comments below. I must admit, the multiplayer was wonky so there is some merit to it.
Hopoo, the original devs, were already making this and got to finish working on it after the purchase. It adds new content and refines the gameplay of the original (movement feels better and the game just generally plays better) - it was also designed with modding in mind.
I don’t think that’s the case at all. Gearbox is developing a mobile game to take advantage of owning the property, along with whatever they plan to do with Risk of Rain 2 now. Returns is from the original studio and is more of a love letter / farewell to the franchise.