Advertising. Their advertisers are probably only paying for views inside the US, so anything they serve outside of the US is throwing away bandwidth for them. Giving away their content still helps their brand if they ever expand to those countries, but even in an expansion there can be different owners involved, among other factors that complicate the whole scenario.

A bigger problem is that if any MTV content is syndicated on other networks outside the US, MTV would be breaking their contract by showing videos to non-US viewers. These types of contracts worked well when everything was based on TV, which has fairly definite boundaries, but hopefully we don't see these types of restrictions in another five years or so.