Up until this last cycle, the Intelligence failed to solve the problem the Leviathan gave it in its mandate. Considering that, why would it bother investing time and resources into other galaxies when it's yet to fix the current one? Also, the Intelligence relies on a ruse in the form of the mass relay network in order to facilitate its plans, which also requires a hub designed to lure spacefaring species. Would it have copied itself and built a Citadel Mk. II in Andromeda? Just how long would this even take? Just how long were the reapers really around that they would have had time to accomplish this?
As for the Leviathan, there's any number of possible reasons that they couldn't leave the galaxy. When the Intelligence betrayed them, they may have lost the resources necessary to even attempt the feat. After all, they rely on other species to do the work for them, species that were undoubtedly wiped out by the reapers. They may have been pinned down for eons, relying on their space pilantirs to observe and occasionally hijack a few minds to help them, but never enough to get the ball rolling for the cuttelfish exodus.