My understanding is that the reason it started replacing other services is that there's a dependency graph that ends up pulling them in, and very early in the boot process depending on configuration[1]. That, and in some cases the available solution did not support their needs[2] (which might have been solved better through upstream work, or maybe not, if the targeted project was slow or unwilling, I'm not sure).

1: https://lists.freedesktop.org/archives/systemd-devel/2014-Ma...

2: https://www.reddit.com/r/linux/comments/6kdcme/why_does_syst...