That's to save space. Searching doesn't search every single post for every word. Instead there is an index of all the words linking them to posts. So reducing the minimum length to three would require adding all the words three letters long to the word list, and linking them to posts in the wordid-postid table.

I think it would only be several megabytes extra, as three-letter words are only three bytes each, and words with over 50000 appearances aren't included (like "the"). But it might be more for some reason, and that's probably why the admins aren't doing it.