GNU Shepherd is an init replacement written in Guile, a dialect of Scheme. Guix ships with GNU Shepherd. Anything that runs in place of init should in principle be small, and hence not too difficult to write and maintain. So you are right that it is not any technical issues, but rather the disinterest of system software programmers in anything other than C, and the disinterest of language X programmers in system software, that is the main culprit.
systemd has had close to 1000 contributors now (as recorded by git), with 30-40 active per month. Higher than I imagined.
https://www.openhub.net/p/systemd
https://www.gnu.org/software/shepherd/
https://www.gnu.org/software/guix/guix.html