Why don't the Zerg ever attack any other Terran faction?

[Pandonetho]

I can't recall much of the lore atm because it's late and I'm tired so I could just be imagining things.

But IIRC the only time the Zerg have ever "attacked" anyone else was during Kerrigan's fuel raid on Moria.

Otherwise, the Zerg seem to like picking on the Dominion/Confederacy a lot.

[Novasquadron]

Well if you think about it.
The zerg could easily whip out the smaller colonial factions.
Where as the bigger struggle would be agianst the Dominion/Confederacy.
I think when you take down the biggest fish, the smaller ones are easy takings.

Also early game SC 1 you had the Confederacy responding to the "Alien" sightings. The Colonial militias were problaby not well equipped to deal with this threat.

But if you want to get into specifics.
I think the zerg is just poking all the Terran factions currently.
To see the strenghts and weakness of each one.

I mean after all any Terran player can tell you.
Zerg is just so d&%! annoying.
Seriously annoying...

[Kimera757]

I can't recall much of the lore atm because it's late and I'm tired so I could just be imagining things.

But IIRC the only time the Zerg have ever "attacked" anyone else was during Kerrigan's fuel raid on Moria.

Otherwise, the Zerg seem to like picking on the Dominion/Confederacy a lot.

Vs the Confederacy: The Confederacy used psi emitters to lure the zerg to "isolated" worlds such as their own colonies. Militarily, putting emitters on, say, Umoja would have been a sound (albeit evil) decision, but the Confederacy wanted to study the zerg, and a bunch of Confederate scientists who keep popping up around alien attacks on Umoja would have drawn a lot of attention.

The Confederacy also had more worlds, and the zerg first appeared in Confederate space. Just through random spreading, the Confederacy would get hit before anyone else would.

Once Mengsk got his hands on the emitters, he would only have attacked the Confederacy, as it was only them who he was gunning for at the time.

Vs the Dominion: Who is in charge of the zerg now? Kerrigan has personal reasons for attacking the Dominion. Also, the Dominion is the strongest force; Moria and Umoja are probably so weak they can be virtually ignored.

[Draco97]

Just to add on to what Nova said, the Combine and the Protectorate have never launched a full on attack against the Zerg (as far as I know) so it makes tactical sense to take out the strongest faction if the other two won't do anything about it.