Systemd-boot didn’t start as part of systemd, it used to be gummiboot (joke in German, it’s what those little rubber inflatible boats are called).
Systemd absorbed and integrated it in 2015.
It did start at RedHat with Kay Sievers and Harald Hoyer, which makes it unsurprising it was absorbed.
I’ve been transitioning to it as my default choice, I’ve never liked grub2, so I defaulted to syslinux for a long time, but lately systemd-boot is even less of a hassle.
The argument was that if you put all your static resources in /usr, you can mount it RO (for integrity, or to use a ROM on something embeddedish) or from a shared volume (it’s not uncommon to NFS mount a common /usr for a pool of managed similar machines).
…that said, many of the same people who made that argument are also the ones that went with making it so systemd won’t boot without /usr populated anymore, so that feature is now less useful because it has to be something your initramfs/initcpio/whatever preboot environment mounts rather than mounted by the normal fstab/mount behavior, and the initcpio/initramfs/dracut schemes for doing that all (1) require a redundant set of tools and network configs in the preboot (2) are different and (3) are brittle in annoying ways.
It still works OK if you’re using a management tool like Warewulf to manage configs and generate all the relevant filesystems and such, but it’s a lot more fucking around than a line in fstab to mount usr once the real system is up like the old days.