Because, so as to make increase the educational value of the degree, you want it to be as relevant to what you are doing as possible. Getting an MBA early on is fine (I did a combined MD/MBA), as it provides great complementary education , but in comparison, the attendings who were in the executive night program at my school were probably getting more bang for their buck since they already had administrative/clinical experience and had much clearer ideas on what they wanted their business training to accomplish.
I'm pretty sure by now I've forgotten most of the accounting and finance I learned first year of b school. I think the healthcare fellowship programs are the best option, in that they are short (1 year) and can be done after completing a full IM residency so as to not skimp on the clinical training.