Why doesn't BURG replace GRUB?

BURG is a one-man fork of GRUB. GRUB is the primary project with an active development community. I don't intend to ever switch Ubuntu to BURG; I would rather focus on improving GRUB.

Look at the commit activity of the two projects (quickest way: bzr log lp:burg vs. bzr log lp:grub/grub2). BURG has only had five commits in the last six months - to all intents and purposes, it's moribund. Over the same time period, GRUB has had just over five hundred commits.

BURG did some useful things with themes, although GRUB has most of that - it just doesn't ship them by default, which IIRC was due to licensing problems. BURG supported a different installation mode which helped some people, but now that GRUB has Reed-Solomon encoding and the ability to skip certain sectors in the boot track this shouldn't generally be needed. There are a handful of other small improvements. None of them justify losing the fantastic GRUB community.

Incidentally, I'd thoroughly encourage reporting the reasons you feel BURG is superior to GRUB as bugs on GRUB.

Because BURG is not ready to be deployed on this many machines. It's very early in it's development, and I'm sure later on, say when they have reached a beta version, people will start to look into it.

Check burgs Project Page on Launchpad to keep up with development(s).

As people have pointed out, the boot loader can be considered the most important part of a system. If it doesn't work correctly, none of your operating systems will start. This is why this type of software has to be extremely mature before it gets deployed on a massive scale.

Colin Watson (one of the Ubuntu grub maintainers) also has more information in his answer.