systemd became the de-facto default init sys because Distro Maintainers have chosen it. I posted this link here before, but I will happily post it again: https://www.reddit.com/r/archlinux/comments/4lzxs3/why_did_a...
Such a thing doesn't happen if the thing offered isn't an improvement over what was before.
As a matter of fact, this doesn't even happen if the thing offered is only a small improvement. We have seen this time and time again in Programmig languages: Exciting, clever new things that offer real advantages vanishing into obscurity because while they were better, they were not better by a large enough margin.
Diversity is great if it offers advantages. Different kinds of RDBMS are a good example. Different kinds of shells. Rust being there as a C alternative, Go being there as a Java alternative, Julia as a possible Python alternative, WASM instead of JS. All great.
But keeping something mainstream just because its different, adds no value. People can and do still use other init systems, but I am very glad I no longer have to wrestle 400-line sh init scripts on my production servers.