Anastasios Anastasatos

Answer: Being regular humans without powers or highly advanced technology, they may not have been able to fight properly against Thanos' forces.

LorgSkyegon

Neither did the Asgardians.

The Asgardians were there and they are superhumans.

lionhead

The average Asgardians are all established to be vastly stronger, more powerful and faster than humans. Their soldiers even more so.

The remaining Asgardians were there.

Anastasios Anastasatos

Answer: Spoilers! Given what we see in Spider-Man: Far From Home it may be that they were offworld doing other vital work.

Answer: Since any reason given would be speculation, the easiest explanation is that they had something else to do in this, the one and only future where the Avengers won against Thanos.