Same goes for you on the second game. It happens a lot of times, when one entry in the series is bad, it makes people lose faith.
Unfortunately I wouldn't say it had two good games, it really only had one (the 3rd). Well at least in my opinion. The first one sold so well because of Xenogears, but couldn't deliver which is why fewer picked up the second and hardly anyone the third.
It's like with the XIII series. People bought XIII because it is Final Fantasy, then when they found out it sucked they didn't return for the sequel and those who had realized Square-Enix did not live up to the promise of hearing the fans complaints and resolving them, so even fewer bought the third.