UED arc was extremely stupid. They appeared out of nowhere like some sort of Deus Ex Machina without any hint or foreshadowing. For some reason they used (almost) the same technology as their wayward bumpkin colonies despite decades of separation. Finally, they conveniently disappeared and never bothered anyone anymore. If they were watching the Koprulu colonies so closely, shouldn't they react in some way to the fact that they were attacked by a race of Ctulhus from another dimension?
According to the timeline it was hundreds of years since the separation between the Terran colonization force and the UED, along with the fact that much of the Terran tech and units are described as improvised, so yeah that's a big problem. But the UED not returning is probably because of the travel time, it'd take decades for a new force to arrive and the entire SC1/2 timeline has to be squished into like 5 years because Blizzard wants to keep all the same characters around looking and acting pretty much the same.
The UED is in kind of an odd place though. The Terrans were just a small colonization sent out made up of convicts (ala Australia) 250 years ago with no further support. And despite this not-Australia being constantly dividing and in civil war with itself it manages to stand up to the fully powerful zerg and protoss forces. You'd expect the united force of earth and its normal colonies (not the one-off convict colonization mission involving a few thousand people sent across the galaxy basically done as a vault-tec level prank/social experiment) to be monstrously powerful and be able to roll in and swat the protoss and zerg like a fly. Granted they already almost did that with the UED fleet (and were doing it until basically everyone decided to help Kerrigan), but there's no indication that they couldn't be sending a much bigger one next.
It's also kind of funny that the Protoss and Zerg are just contained within a small sector of space while as far as we know the UED is the actual galaxy-spanning civilization that could be fighting any number of other races completely unmentioned in Starcraft.