My recollection of prior HN discussions is that a large part of it was that it involved interfacing to systems in a large number of agencies or departments. Most of those systems were old systems, independently developed systems, that had not been designed to have APIs for other agencies to use (or had not been designed to have APIs at all).

I think it might also have had to interface to state systems in the states that were not running their own ACA exchanges, and to the systems of numerous insurance companies.