"Resources" Or lack therof?
The Reapers not going to Andromeda could boil down to programming. The Catalyst was designed to monitor the Milky Way, so its own coding might prevent it from having an interest in affairs outside its parent galaxy. That is of course assuming that the Reapers never went to Andromeda. It may turn out once Andromeda ships that the Reapers were active in the entire Local Group, and they're the reason why the Remnant became the Remnant.
The Leviathans not colonizing Andromeda wouldn't necessarily be implausible either. The Milky Way is incredibly vast, with between 100 and 400 billion stars, most of which are likely to have orbiting planets. There would be no compelling reason to leave on such a long and dangerous intergalactic voyage, when your parent galaxy provides more than enough resources than you'd ever need and with many more planets than you could probably ever colonize. If the Leviathans did go intergalactic, there are also other galaxies closer to home than Andromeda. The Magellanic Clouds should be the next stop after the Milky Way, if exploration and colonization is the goal.
On that note I'm hoping the Reaper War is the reason why the Milky Way colonists set out for Andromeda. I think that works as a more compelling reason to go so far from home than exploration, particularly when less than 1% of the Milky Way was explored by the current cycle and the Magellanic Clouds are closer.