Long distance communications in the Republic/Empire often seem "fuzzy." Possible in the past it was worse, requiring representatives to have autonomous decision making authority. As communications somewhat improved they simply maintained the practice. Being a Jedi required possessing a natural ability, they couldn't be recruited from the general population. Rising to the Jedi council could have been (at lest partially) dependant on how powerful you were with the force, a determination a "outsider" couldn't make. In terms of percentages, they spent less of their known history in civil war than America, Britain and quite a few other nations.
This was an epic fail. They ended up giving an important vote to a simpleton who was actually a senator. And that vote was the beginning of the end. They gave the senators too much power who gave their chancellor too much power. But going from democracy to an evil, imperial dictatorship is the trump card. The civil war did only last for a few years, but it resulted in something worse than a civil war. One thing I'll give credit to the Federation for is that they're so self aware of their freedom that they would immediately catch on and try to stop anything like that from happening. But that's right now with 150 members. What happens when or if there are too many star systems who aren't as concerned when something affects another member that's far far away? That's what happened with the Republic. One member was invaded and basically nothing was done about it because the senate was too bloated and corrupt to agree to immediate action. The same thing might could-- possible-- maybe-- happen to the Federation if it expanded too far and became overloaded with members and alien senators that feel too distant from each other to feel like a real community. Some of those feelings came up in the Maquis situation.
Or Tasha's homeworld. It's exact status is unknown, Federation member, Federation colony, Earth colony, completely independent colony. But on the surface it doesn't sound like outside help came flowing in when they developed trouble.