In The Avengers, the main plot is that Loki is trying to conquer Earth on behalf of Thanos so that the latter can get hold of the Tesseract and put his "restore the balance" plan in motion.
But in later movies (Guardians of the Galaxy, Avengers: Infinity War) we see that Thanos was perfectly capable of doing that himself, or sending Nebula and Gamora to do it. He never really needed a complicated plan like the one Loki was scheming to get the Tesseract.
Moreover, we see that he is perfectly capable of moving his army from one part of the galaxy to another in a timely fashion. So why did they need the portal to bring the Chitauri to Earth?