Both matches showed Sampras's mental weakness (up to that point).
In the 1992 US OPEN final, he was leading 6-3 4-6 6-5 and serve. He played an awful game to lose his serve and also lost silly points in the tie-break to lose that third set. During the fourth set, it was not that he tanked, but he clearly did not dig deep, he was kind of content to lose that match in that moment.
In the 1993 AusOpen SF (against Edberg also) he was leading 4-0 (or something like that, two breaks up ) and he managed to lose that huge lead to lose in a tie-break. He also had chances to win the third set, but again lost his chances and played very flat to lose another tie-breaker.
After those two losses, many people started to talk about Sampras, that he would never be a GS champion again, that he had not the mental strenght to win important matches, that he had a great complete game but not a champion mind (it was not that bad to lose against Edberg two close matches, it was THE WAY he lost those matches, the way he squandered important leads, playing awful and apparently careless in the most important games, his mental weakness is what drove people mad about him in those matches).
Few weeks after that he got to nº1 (he had chances to become nº1 several times months earlier, during the last months of 1992, but everytime he lost the important matches, kind of like Federer during the summer of 2003 ) and people criticized him even more.
But then he won Wimbledon, US OPEN and the Australian Open back to back to back (first player to win 3 consecutive GS since Laver) and so people realized he had changed, he had become mentally stronger.