Why didn't the reapers go to andromeda?

Because Shepard wasn't there.

Why would they want to? Their goal, as stated by both their controlling intelligence and the people who made it, is to prevent galactic  wars between organics and synthetics in the milky way. Intergalactic things isn't their jurisdiction.

The Reapers were made by the Milky Way, for the Milky Way. If they're gonna go harvest Andromeda too, they might as well go harvest Triangulum, Messier 32, and all the galaxies near there...which would probably end up meaning in the billion-something years since they were created, they probably wouldn't have returned to the Milky Way once. 

Given their arrogance on the whole thing, they probably assume all the other galaxies naturally made their own Reapers too. 

Wasn't in their programming!

because the writers didn't want them to

My guess: they tried to but were defeated long ago, possibly by the Remnants. The Remnants may mention this or you will find this data somewhere. They laugh at Reapers. Ryder may encounter parts of Reapers that were defeated long ago.

Or, the Remnants are remnants of Reapers.

I've thought about this and some possibilities may be that:

1. The races of Andromeda have not yet reached the point in which the Reapers like to harvest.
2. The Reapers had not gathered sufficient data on Andromeda before attempting the costly trip and harvest.
3. The races of the Andromeda galaxy are or were too advanced for the Reapers to comfortably challenge them.
4. They have and we just don't know it.

This brainstorm without the benefit of knowing much about the Purple Squid Cobra Cat People, Kett or other sentient races of Andromeda.

Because beyond your comprehension™, silly.

1) Why should they? They are programmed for the Milky Way, so unless anyone out of Andromeda endangers life in the Milky Way, it's not likely that their programming would "care", so to speak.

2) If you don't believe in 1): Do we know that they weren't? In the Milky Way, the writers have windows of 50 thousand years to tell stories without them. Same could be true for Andromeda. Maybe they showed up 30k years ago.

3) If you are speaking of in the game: when we arrive in Andromeda they might be destroyed by Shepard or are not programmed to harvest civilizations anymore.

it wasnt in their programming, they were only looking out for the Milky Way civilization to "preserve" for their creators, the leviathans, so they could keep their precious slave races.

Because they stay in darkspace planning for the next harvest in the Milky Way. After each harvest, Harbinger has a team meeting with all reapers to see what they could do to make the harvest more efficient. He reviews some of the ways the reapers destroyed the enemy. For 50 000 years, he comes up with better ways to destroy the organics. So he's not thinking about another galaxy.

I would like to have Harbinger in Andromeda. ASSUMING DIRECT CONTROL. He's one of the best characters in the trilogy. I'm sure he will be seen again when Bioware remakes the trilogy.