Well, the Klingon and Romulan Empires are Empires. They conquer and subjugate other sentient beings. If the centers of their Empires collapsed it would be the signal for their conquered territories to rebel and fight for their independence again while the military would be too busy dealing with whatever destroyed the Homeworld to keep things under control. With the Feds, it's a multi-species cooperative and not an Empire. So losing one of the Founding Worlds (there are 4 others) wouldn't be SUCH a blow by comparison.