BURG looks a lot better and user friendly than GRUB, at the same time it get the job done.
Why Ubuntu have not switched to it?
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.
Another, more current reason why BURG might not have been switched to is that it appears to be an abandoned project. The last commit to trunk as of this writing was all the way back in 2010.
Without continued development, new features and - more importantly - bug fixes to make BURG work better with newer hardware and software simply aren't happening. Unless development resumes or the project is forked, there's little chance it will be accepted.