As explained elsewhere on the forum the main reason we’re using Mailgun is for future functionality that we’re building into Ghost to give you insights into audience engagement and analytics. Currently no other bulk provider offers similar access to what we need via their API so we’re starting with Mailgun. Members is in beta so things are being kept as restricted as possible so that core functionality can be built out without a lot of the over engineering needed to support adapters for multiple providers.
Bulk mail is also very difficult to do properly without damaging sender score/reputation at the scale our target audience will be using members with. Sticking with a provider that properly handles bulk email (not transactional, or ad-hoc smtp) removes a lot of those issues for the much larger non-technical audience than the smaller but vocal minority of technical users.