Milky Way Ahead of Andromeda in Terms of Development

I'm going to go out on a limb here and say that Andromeda will have races on the Prothean level of advancement.

Its possible the region is simply not very heavily settled by anybody, and the "natives" don't have a major numerical advantage because they're just frontier colonies too. If the area has no mass relays, they wouldn't necessarily be in a position to call on mass reinforcements quickly, and the conflict might not be considered a high priority.

Also, Bioware generally hasn't emphasised technological advantage leading to military advantage. For example, the human victory in the First Contact wars, or the emphasis on Turian and Krogan warrior prowess over Salarian and Asari technological. Even the Reapers don't seem to have enjoyed all that great an advantage.

Don't forget that the Milky Way has been harvested every 50000 years by the reapers for god knows how long. The Kett and other species probably didn't have that happen to them, so it's possible that they may have used up a lot of resources over the millennia.

Technology wise the Milky Way had access to technology that jump started their species evolution and propelled them forward by centuries, again thanks to the reapers. Andromeda probably had to figure all of that stuff out themselves, so the tech gap may not be that large by the time the arks make it there.