Board Thread:General Discussion/@comment-4738647-20140421191003/@comment-1330470-20140421204437

I already gave you (what is most likely to be) the reasoning. It is not a technical thing.

It's just that such short URLs are in short supply, and Staff wants to save them for when they're really justified to use. I do not know when this rule came into practice - perhaps it has always been in practice - wikis that currently exist at short URLs probably got such URLs by appealing to Staff. The rule can be overridden if Staff thinks it's justified.