it's not really the bootloader, actually, it's just the partition table, e.g. gpt_both0.bin file. Android has its own partition scheme where it needs a bunch of partition (userdata, system, cache, recovery, persist, misc, modem, ...), for linux we create a simpler partition scheme to avoid splitting the eMMC into un-needed partition and wasting space.
It is true that userdata might be ~5GB, but that still wastes quite a bit of eMMC. That's why we have this very simple partition scheme for linux with only 1 large "rootfs" partition (of course we kept the mandatory partitions that are used by the bootloaders). and if i remember correctly, the 'rootfs' partition is almost 7GB. So in fact it was a conscious decision to have different bootloaders, and that won't change.
While we will keep it that way for the Linux releases, if you really need/want to use the Android partition scheme , that's doable (and not difficult). You can flash the linux rootfs image .img file into userdata, and update the boot.img to use '/dev/disk/by-partlabel/userdata' instead of '/dev/disk/by-partlabel/rootfs'.