It does seem that Final Fantasy Tactics/Tactics Advance were not that popular compared to other Final Fantasy games. I really liked Tactics, but I think that maybe Tactics might not have been as popular is because there were very few characters besides the guests. There were mainly generic characters and secret characters. I thought that the only real character that developed was Ramza. Another reason why Tactics might not have been as popular is because the battles got really repetative. It was really annoying when you had the characters exactly how you wanted them and you still had to keep fighting battle after battle. These are about the only downfalls I can think of. I can't really say much about Tactics advance because I haven't beaten it yet. I stopped for a while and then I lost it.
Maybe people didn't like the laws. I know that I really didn't like the story for Tactics Advance.