No one took him seriously because his theory had numerous problems that Aristotle's didn't. It was known during Aristarchus time that the earth is about 24,000 miles around. If the 24 hour cycle of day and night is caused by the rotation of the earth, that means that the earth has to be rotating at 1000 miles an hour. This means that if you jump up in the air and stay up for a couple of seconds, you will come down several hundred feet from where you jumped up. Many centuries later, Galileo did come up with an explanation for why this doesn't happen , but Aristarchus could not account for this, and Aristotle could. That's just one of many things wrong with Aristarchus' theory, which is why the majority of natural philosophers accepted Aristotle over Aristarchus.

Aristotle is actually the father of modern science, not the enemy of it. He did not have the concept of a laboratory experiment, but he was the first to recognize that science required observations (Unlike Plato who distrusted the senses, and wanted to reach knowledge only through deductions.) This is another reason that Aristarchus' theory was inferior to Aristotle's. Astronomers still can't do laboratory experiments: you can't say "let's blow up a star and see what happens.".Ptolemaic Geocentric astronomy, like modern Astronomy, was based on observations, and it had thousands of observations backing it up. There was no reason at the time to assume that throwing out those observations would accomplish anything.

In Copernicus' time things were different. Ptolemaic astronomy was in state of crisis, because it was encountering data it couldn't explain, and consequently there was a need for a new theory. But all science goes through crises eventually, and Ptolemaic science is not pseudoscience just because it was eventually replaced by something else. By that criterion, Newton would be a pseudoscientist, because his ideas were eventually replaced by Einstein's.