Because under the hood x11 code is such a hacky mess the developers found it easier to start from scratch than add new features like HDR and VRR to x11.
If new features don’t matter to you, there’s still plenty of distros and DE’s and WM’s that ship x11 and will continue for a very long time.
I don’t think it should matter, at least not until we’ve guaranteed everyone their human rights. Nutritious food, safe shelter, clean water, medical care.
I don’t think we can afford to worry about luxuries until we solve the problem of affording people.