In this model, I am thinking feudal technocracy versus participatory democracy. As both polities grew in population and territory, the former began sliding into oligarchy and the latter into an unmanageable mess. So they ended up talking each other into meeting in the middle -- perhaps with a form of representative democracy where each level of representation (local, regional, continental, planetary, system) selects who will represent it at the next higher level, culminating in the President
Pro Tem or Secretary-General or whatever of the interstellar governing body.
In lieu of conventional proxies, this Senate/Parliament/Congress/Assembly/whatever uses a ranked-choice voting methodology, with each representative getting a number of votes equal to their mainworld's Pop digit, to spread around as they like in support of or opposition to resolutions before the body. This vote-splitting should also create a dynamic political forum without our relying on multi-world political parties to keep it lively. Votes of No Confidence may be called at any level of this RepDem hierarchy, except the very bottom, where elections are scheduled to recur on a regular basis.
Just a (rough) thought...
Economics. More efficient, more productive, more stable, more resilient. One market, one currency, one set of regulatory standards. Higher quality of life for all concerned.
See also one of my favorite thought experiments,
Ken Pick's monograph on the qualitative shifts implied by Tech Levels, which suggests that it is a mistake to try an advance beyond TL11 without a functional, ubiquitous interstellar government. (It takes a robust, developed society to accommodate things like PGMPs and AIs, yet remain standing for long.)
Hence the MolaFed advancing on to early TL12, while its (frankly, behind the curve) neighbors are just now barely able to climb out of their own gravity wells without relying on -- Sol help me --
rockets, for Goodness' Sake.