One of the main questions to come out of the fourth season episode "Asylum" was why would the Kromaggs make a treaty with humans...the very race they have sworn to destroy?
I would like to propose this theory based on what was said in my discussion of "The Other Slide of Darkness." If the Kromaggs started out not having the ability to control the time they had once they slid to each earth, then it may give us an answer.
Imagine for a moment that Kromagg invasions before meeting Quinn's double (c. 1995-1996) took place by setting a time limit for that earth. They would move in, take over the world by force, grab the resources, and slide back out to their newly adopted homeworld thus gaining more strength and knowledge for the push to get back to their home. But, things wouldn't always go as smoothly as they liked...
Take the "Asylum" world for instance...when they reached this earth, they came across unexpected resistance which held them back. As their time to slide back out creeped closer, they were forced to cut their losses and get what they really needed so the venture wouldn't be a waste...the resources. So, even though it repulsed them to do so, they made a treaty with humans who would give them those resources in return for "peace." The Kromaggs only agreed to this because they knew the humans would get their peace by default...with no way to return to that world and their time limit running out, the Kromaggs were about to be gone anyway. So, the Maggs took what they could and left...thus leaving the world in turmoil that we found in "Asylum."
There is also another theory concerning the Kromagg forces...even if they did have the ability to come back and didn't have the time limit, what if their forces were not built up enough by that point to significantly quell the revolution on that earth? We know that a large percentage of the Kromagg population was killed upon the exile from their homeworld and they have not been able to readily repopulate due to the weapon's side effects. So, how strong can their forces truly be? We are given another indication of this in the "Common Ground" Online Slide...the Kromaggs state that they only have the ability to conquer five worlds a year...five?!? That doesn't denote very numerous forces when you take into account that not many are needed to conquer a world thanks to their usually superior technology. Once again, this scenario would end with the Kromaggs realizing they couldn't beat the people of the "Asylum" world and so they cut their losses, got what they could, and left before they lost the advantage of fear.
Both are something to think about...