Why hasn't Xbox obtained KSP?

It was sold to another company

This is a posr on the Kerbal Space Program forums that came from an official Squad member on May 25th:

Quote: "We’re currently working on an updated console version of KSP, and are removing the game from the Xbox and PSN Stores until it is complete. This will be a free update for anyone who already owns the game on Xbox One or PS4"

Source:https://forum.kerbalspaceprogram.com/index.php?/topic/161042-ksp-on-xbox-one-currently-not-avalible/

Hi! Unfortunately there hasn't been a way to obtain this achievement since and it doesn't look like this will change in the future.

Thanks for letting us know, RickyR1998!

If you're seeing lag/framerate issues when playing an Xbox One title, the
Xbox One Slow Performance Troubleshooter can walk you through steps that can help.

Make sure to use the ✔️ and ❌ buttons at the bottom of each page to work your way through the entire guide!

if the game developers allow mods and add in a menu for downloading and installing them then it's all good, but if they don't, attempting to mod the game could result in an Enforcement Action against the Xbox Live profile as this is explicitly against the
CoC and TOU.

you could try contacting the Developers of KSP asking if they have plans to add in User Created Content for the Xbox version.