I don't think it's anything to do with control and lockin, it's to do with the fact that Debian feel they have a far superior package management system and allowing other, inferior (from their perspective) systems is a Bad Idea which they'll fight tooth and nail every step of the way.

It does seem silly from a rubyist's perspective but their approach does make a heck of a lot of complicated software work together in a way that makes administering complex server setups a doddle compared to how it could be.

There's just a yawning philosophical gap between how the Debian and Ruby communities look at the world and approach software management.