There's the snarky answer, and what I suspect is the real answer.
First, systemd and everything associated with is just so kewl and shiny that's it's a privilege to even use any of it, which makes it all the more amazing that they're actually welcoming us to do so, instead of making us fight for a place in line.
Second, X11 goes way back before anyone was really concerned with security. I suspect from a core competence point of view, the X11 coders are far more comfortable and far more engaged with the graphical display code than the input side. I get the impression that a lot of effort was spent in properly cleaning and separating the root-requiring functionality. I know I've read of KMS and DRI work for years now. It's been a long road, and I believe it may have only been in the past year that the display side has gotten to the point where they could think about going rootless.
I also suspect that the input device part is not their core competence - they'd like events coming in from "elsewhere" and get back to their graphics work. So along comes systemd, saying, "We'll handle the gnarly details of console access and security for you," and X said OK, if only in the spirit of modularity and going back to their graphics work. (Graphics work includes processing the inputs, not just drawing outputs - I think they'd just like the inputs to be clean and handed to them.)