Why didn't an Asgardian go to Earth sooner?
In the beginning of Avengers: Infinity War couldn't they teleport someone to Earth sooner? When did they realize the ship belonged to Thanos?
If he did announce it early enough they could have teleported to Earth before Thanos attacked them. Was Loki willing to help his people?
If he was he could have revealed that he had the Tesseract and even though they didn't have the contraption they had in the first Avengers movie couldn't they use it as a group as in Guardians of the Galaxy?
Or give it to someone like Thor (or the Hulk, who in Endgame was shown to be very strong, if Loki was untrustworthy and Thor didn't want to leave his people, or maybe Heimdall) to go and warn Doctor Strange?
If they warned him early enough they could possibly have 4 Stones (the Tesseract, the Eye of Agamotto, Vision's Mind Stone and, since they knew where it was, the Reality Stone).
Given how dire the situation of the Asgardians looks at the beginning of Infinity War, I think this is simply a matter of no time to do it, because Thanos hit them fast and he hit them hard.
For starters, if you remember Ragnarok's mid-credits scene, you'll see that both Loki and Thor seemed awed by the ship they were seeing. They're surprised to see it (to say the least), and they just recovered from the Ragnarok battle - not sure they're already fit to go back to battle.
Next, they don't know who's? attacking them. The rescue message only mentions they're being attacked.
Asgardian PA: This is the Asgardian refugee vessel Statesman. We are under assault, I repeat, we are under assault - the engines are dead, life support failing.
Actually it also mentions that the ship isn't combat-ready. As Heimdall, the only person able to teleport someone to Earth, is on of the few soldiers it has, it's not a good idea to waste his fighting time sending another fighter away; people are going to die while he does that.
Our crew is made up of Asgardian families, we have very few soldiers here. This is not a warcraft. I repeat, this is not a warcraft!