I can give you a better reason: the relational model is the most powerful adhoc querying system ever created. The output of a query is just another table, which can itself be queried; this recursiveness is not present in any object system I've seen. When an object database can query several classes of objects and present you with a brand new class of objects, they'll start to have something interesting. Until then, relational databases will rule wherever adhoc querying is important...ie., most businesses.

That said, for specific applications, alternative databases systems can do very well. For business applications, the relational database is usually the main point of it all, and websites are just front ends get data in and out. But for some projects, the website is the whole deal. For those, there are sometimes options to get dramatically better performance or scalability by using alternatives...object databases, graph databases like Neo4J, or distributed systems like Cassandra (runs part of Facebook), Scalaris, Google's Bigtable (available on Google App Engine), and Amazon's SimpleDB.