Because this board probably uses MySQL version 3.23.x (the latest stable). 4 characters is the minimum.
See
http://www.mysql.com/doc/en/Fulltext_Search.html
Full text searches are also very limited with MySQL.
WHERE blah LIKE '%blah%' can't be used because that would take way too long on big tables.
MySQL 4.0.1 or up can do boolean full-text searches on huge databases but it will probably take a while before they release a 'stable' version.