A company as large as Microsoft has resources to do a lot of things, but you’re not borrowing resources from the Office team to help on this project.

The relevant measurement is the resources Mojang has as a studio. And I expect the decision here is that they don’t want to commit to the long term maintenance of three renderer implementations on the Java side.

Another concern is that modding is a major part of why Java Edition is so popular, and that includes shaders specifically. This is already going to cause chaos in the modding world as it is, no need to compound that by making shader mods that much more burdensome to maintain.