Hi Nick
Because MySQL is free, at least for non-commercial applications, while Oracle certainly isn't. Also, the native MySQL support offered by PHP is quite extensive, whereas the native Oracle functions aren't quite so comprehensive.
Those would seem to be the reasons IMHO.
However, there's absolutely no reason why you shouldn't use Oracle and PHP.
Btw - there's a thing called ADOdb which is a DB abstraction layer for PHP such that you can use exactly the same code to query any database - you only have to change the connection parameters. It's available at http://php.weblogs.com. There's a tutorial and documentation there. Also, it's just an extensive class, so you don't have to reconfigure to make it work.
HTH
Norm