Third-party services are a lot of work and are also generally high-maintenance - 
Nepeo has done a ton of work for the Mobile Ad plugin and it requires regular updates too. We don't have the resources to cover all possible third-party services, especially when pretty much every individual user suggests different services that we should support. This is part of the reason we have an addon SDK, so other developers can pitch in without having to wait on us. Meanwhile in practice many developers continue to publish to the app store anyway, so the pragmatic thing to do is to add features to support that, which we have done with Mobile Ad.
Remember there's a big difference between not having the resources to do something, and not being interested in something. I'd love to do hundreds of suggestions and integrate hundreds of services. It's simply not feasible though.