While CMS pays crap for cardiac anesthesia, they pay big bucks for open heart surgery, hence the former has strategic importance for hospitals and anesthesia groups. Pain doesn't.
Also, a cardiac fellowship will make one a better OR anesthesiologist. Pain won't.
The problem with Pain is that, if the person is unhappy with the post-fellowship job, it becomes a wasted year. It's like a fellowship in Palliative Care or Sleep: it's only worth it if one is happy practicing it. It's not really useful in the OR. It's a much bigger bet than Cardiac.
Maybe I am missing something...