Consistency and familiarity would both be good reasons to me.

Non camelCase variables and CamelCase classes in Java for instance would really feel out of place to me, whereas using the same case conventions for some ruby code would feel equally 'wrong'.

When it comes to conventions like this, I don't think there's a substitute for having a "when in Rome" attitude. If you're writing some greenfield code, Rome is all the other projects in your language of choice. If you're adding some functionality to an existing piece of code, that existing piece of code is Rome.