Why haven't Andromedan species come to the Milky Way?

Leviathans had AI problem in their galaxy, not in Andromeda. Dealing with AI in entire Universe is impossible and setting the border in "x galaxies away" would be more artificial than limiting it to MW.

Again, I think what Iakus said was technically possible. Bioware chose to not go in that direction though.

How close is the Andromeda Galaxy to ours?

Technically maybe they did send a ship here, the only problem isn't why they did it but how long ago it happened?

My point is what if they did and a descendant of a survivor or two have been planning a means to return home?

Would need quite a bit of help the kind that's hard to hide and as mentioned elsewhere a practical reason for a trip to the Andromeda Galaxy?

Discovering the remains of one of their ships, recoverable data revealing evidence of the Vaults a resource worth even more than the "Golden Planets" in the Helios Cluster?

BUT that information is far out of date so could one of the races we know about from Mass Effect is descended from one of those who embarked on the trip from the Andromeda Galaxy?

I recall another series where an alien ship arrived in orbit of Earth with a sizeable population of slaves governed by overseers most of who either died or disappeared into the population as they settled on Earth so maybe the same here and the majority of these people have no interest in returning BUT someone certainly does so who would you pick as descended from Andromeda?

I had always secretly hoped that our fight versus the Reapers would take us to Klencory. If we're going to use a MacGuffin, at least make us find it, right? Being told about "the prothean device" on Mars was a tremendous but expected let down, for me.

maybe the races of the Andromeda Galaxy started the travel to the milkyway galaxy but they detected with sensors, or saw through the windows giant space objects the reapers floating there or returning to Darkspace and got destroyed or if they didn't get destroyed, they decided to turn around and head back to the Andromeda Galaxy

Interesting idea, i would not be surprised if something similar is in MEA. 

There are more dangers to a civilization than Reapers. You can't assume that just because Andromeda may not have a Reaper infestation to deal with, that they've had the opportunity to culturally evolve.

A good, old fashioned meteor strike could send a civilization back to the stone age. War. Environmental changes. Or they just start to crumble under their own weight.

I'm assuming that the Remnants are probably more advanced than we are, but we don't know their story. Something horrific may have happened that has them terrified to leave their home base. Or some cultural taboo has taken shape over the eons.

They sent someone, but they got reaped by der Reapers.

There is a couple of different reason why they may not have visit MW. (Ingame reasons ofc.)

They may not have felt the resources are not worth the dangers/cost (As a couple of people here on BSN already have said about AI)

The races in Andromeda may have never got to the level of technology before they got wiped out. (MW-races only are this advanced because the reapers)

If the races did reach a high enough level of technology to travel to another galaxy, they may have picked a different galaxy.

I too seen that trailer and when I saw that imagine I paused at that imagine and I was like is that a reaper or a leviathan, but don't know will just have to wait till the game comes out and find out for ourselves

The Andromeda inhabitants probably never had the tech to come to the Milky Way, they don't seem to have any Mass Effect technology like the relays. Heck, maybe they only had a recent jump in their technology thanks to some stray Reaper.