Because they can.

• OpenRC just feels nice
• Runit is simple
• S6 is really fucking fast
• Some distros (e.g. Guix, Void, Gentoo) come with non-systemd init systems by default, but I use them for other reasons

As for why I sometimes use musl, I like BSD. Also, Alpine Linux uses it by default, and most glibc software I’ve tried works just fine with gcompat.