The database model behind WordPress doesn’t follow several good database design rules and conventions. When we’re designing a database for a specific purpose, knowing all its desired functionalities in advance, we can follow all of those rules. But WordPress needs to cover anything that anyone could have in mind, so sacrificing foreign keys and using EAV is something that must be done.

http://www.vertabelo.com/blog/technical-articles/wordpress-behind-the-scenes-part-2