It was messy (to a certain extent it still is). Reasons for GRUB(2) are mostly UEFI, multiboot support (eg: bsds, windows nt derivatives like modern windows). Grub fails the test of making simple things (as) simple (as possible). But it does support booting to space invaders. So there's a trade-off there, and I agree, it's not entirely clear much was gained from moving off of Lilo...