StereoKit and MRTK are currently part of the same team, so these people are my co-workers! :) We swap techniques, code, share design docs, etc. StereoKit joined the MR Middleware team towards the start of this year, and over time you'll likely see StereoKit move much closer to the Microsoft MRDL (Mixed Reality Design Language) and pick up similar features.

why build two toolkits that are similar?

MRTK is more of a library. This seems like a stand alone engine.

Yeah, this is correct! StereoKit is an XR App specific engine, which allows it to make API and architectural decisions that are very different from those a general purpose game engine would make. It's a completely separate alternative to Unity or Unreal.

MRTK for Unity benefits from the support of having the entire feature set of Unity with it, but it also comes with the complexity and knowledge requirements of a complex game engine. Game developers may like this a lot, but it is a lot to take in these days, and is way overkill for many applications.

StereoKit benefits from a simpler XR focused feature set, app focused design, and better integration with the normal C# ecosystem. It's basically just a Nuget package, so it's much more familiar to developers that come from a normal C# background, and can benefit from other libraries on Nuget! There's a lot of other details here too, I've done some pretty detailed talks that get into the weeds of the subject, should be able to find a handful on YouTube :)

You can use Nuget packages with Unity & MRTK, I have not had issues there

You can with some work but it's not really officially supported.

You end up with issues where the nuget package will not have meta files, will have multiple targets that unity does not support and will take tweaking to get things compiling again.

https://bobsutton.typepad.com/my_weblog/2012/07/dysfunctiona... (Historical Microsoft)