It's been a while, but IIRC at the time Rails got started MySQL actually did not even support foreign key constraints. Since that was the DBMS of choice, it wasn't much of choice.
InnoDB's foreign key support predates the existence of Rails by several years. However, InnoDB wasn't the default storage engine for MySQL at the time, so that may be a factor.
I wonder how different the state of database application development would be today if all those cheap whitelabel webhosts powered by cPanel or Plesk (where most of us got started, I imagine) opted for PostgreSQL instead of MySQL - which would have influenced the major MySQL adopters like phpNuke, phpBB, WordPress, etc.
