The early 90s were so stacked with many talented players while the courts were so diverse, that you just couldn't really call most outcomes of the tournaments prior. Styles and personalites were clashing each week around the world. Stich, this might sound silly, but he got married too soon and therefore he didn't put much work in his career. He was like only 22 when he got married. Now imagine being married to a 22 year old actress who wants to enjoy life.
Stich could have won more majors between 1991 and 1995 had he put more physical effort and work on his mental strength but he was lazy to say the least. But then again facing the likes of Courier, Bruguera, Muster, Chang, Medwedev, Agassi Berasategui, Gustafsson or Kafelnikov at the French on clay is hard enough anyway. Same thing for Melbourne, you got guys like Lendl, Edberg and Courier who did well there all the time due to the slower hard court conditions and then add Becker, Agassi and Sampras (maybe Chang and Krajicek too) in the mix and you got a field that's very strong already. Or as for the US Open you reverse the roles and put Sampras, Agassi, Edberg, Courier as serious contenders while adding and Becker, Krajicek, Chang, Ivanisevic and Lendl to the equation.
Ironically the toughest and most stacked Slam, as far as competitors goes, was the place where Stich had his most success: Wimbledon. His main rivals used to be hard hitting serve and volley players, just like he was and even guys like Lendl or former champs like Mac still gave it their all when playing there, with decent results. Agassi played better and more focused in Wimbledon at times as he did in the FO or even the US. Ivanisevic was becoming a force on grass, Courier did really well between 1991 and 1993 and even Tod Marting had solid results then. Edberg was naturally the better grass player, while Sampras and Becker were just too hungry for WB and hard to beat, except for that one time.