Understanding systemd-boot
The first thing to note here is the name, systemd-boot. It's shocking, I know. We have a systemd component whose name does not end in the letter d. But seriously, systemd-boot is a component of systemd that has some cool features. Compared to GRUB2, it's lighter-weight, easier to configure, boots faster, and works well with the modern implementations of Secure Boot. Contrary to popular belief, systemd-boot is a boot manager, not a bootloader. It can automatically probe for other operating systems on the machine and add them to the boot menu. (GRUB2 only does this when you first install the operating system, while systemd-boot does this each time you boot the machine.) Once you've booted the machine and have chosen your desired boot option, systemd-boot hands the boot operation over to a real bootloader.
So, why isn't it more widely used? Well, it's just that systemd-boot only works on EFI/UEFI systems. There are still a lot of...