Why was Data never promoted?

I agree. In fact, it was almost as if everyone forgot about the possibility of promoting Data. As an android, he probably did not feel slighted by that, but it was very unfair.
If they could have put Data on a ship with other androids, he would have been fine. But, he was practically unique.
He might have fared better on Jupiter Station, where they had a cybernetics lab.

 

I don't think either the incident in Pen Pals had a serious impact on his career. If it had been the case, everyone in the senior crew would have been demoted to ensign before the end of the 7th season.

Obviously, you have to wait longer before being promoted full Commander than being promoted full Lieutenant, except if you're a rising star as Riker or Shelby.