Because it'll be a cold day in hell when I learn LUA
No, seriously, I think it is much easier if we can just create GUI's in HTML/XML and then load them via some core engine command rather then forcing people to code GUI's in LUA. I feel GUI's should be data-driven and not hardcoded.
Besides, you can use all functions in LE right now in any programming language you wish without restriction. I'd hate to see that change.