No relational database sort of hidden under an object-relational mapper, a template language where the template is valid HTML are two reasons that pop out promptly. Also, Python would be more popular now, perhaps taking some space from things far uglier than Python or Ruby.
Just imagine the joy of using ZODB instead of EJB's layered on top of Oracle databases.
I don't understand why having your template be valid HTML is even a little bit good (I'm not saying it's bad — I just don't see why it's better than anything else). I don't think sending unprocessed templates as pages is a common use case, so there is zero benefit in having them be in a language that a browser can render unmodified.
Just imagine the joy of using ZODB instead of EJB's layered on top of Oracle databases.