For the most part, we're loathe to reinvent wheels (honestly, that whole "Perl on Rails" thing was due to a long, complicated mess we had little control over). We also have legal restrictions regarding accessibility which may very well have played a part in this (just a guess, but I wouldn't be surprised). You can read full explanation of our reasons at http://www.bbc.co.uk/glow/docs/articles/what_is_glow.shtml