In the Man-Kzin Wars, a series of books written by Larry Niven, the Kzin had an extensive empire that spanned many star systems by the time they encountered humans. Their technology had not progressed much in centuries because of in most part bureaucracy.
They had no FTL flight or FLT communications. All the new technologies came from the home world and expanded out to the fleets/armadas at the speed of light, if it was a digital patch, or at the speed of large cargo tugs which had a maybe a .25C max usable speed with maybe a 1C total delta V available to them before refueling.
The end result was, they didn't change technology much because if they did so on their own, ship parts would not fit each other's fleets when they came together as they did time to time. Since these were generational war fleets, you had to mix the gene pool once in a while.
Any way it was explained better in the books than I can here, but the point is, the aliens in ID could have a similar problem if their empire was vast enough and possibly poorly understood technology which couldn't be changed on the fly without major rework, which then would no longer conform to the rest of the fleet when they get back together.
Also ID didn't make as much sense as to why they were here in the first place. Yes we have a good sized iron core, but Mercury has an outsized iron core and much to mine due to how much thinner the crust is. You go to a class M planet for the thing that class M planets have, namely a biosphere, yet they didn't seem to care much about the condition of said biosphere when they did arrive. At least in V they wanted the planet and its food sources, and took steps to not destroy the biosphere.
Rezulin