To relegate an incredibly complex issue to simply "quality of players" and "money" you'd have to not be taking the debate very seriously at all. Trolling wouldn't be an unfair accusation.
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Now for my attempt at a serious answer. If Liverpool fans will allow me.
Any one league campaign can be explained relatively simply. Any Liverpool fans can readily tell you two or three reasons why last season did not yield them a title, and one of them would be Manchester United, for example.
But to discuss a period stretching as far back as 1990 introduces all manner of complexities and variables. Liverpool have had many managers, dozens of new players, and a change in ownership and capital structure, never mind what's happened to all the other clubs in the league, and indeed which clubs they are.
Wigan Athletic are a prime example of the inability to pinpoint any factors to explain a club's fortunes over such a long period of time. Relegated in 1993, winning the Third Division in 1997, three failed playoff campaigns before winning the 2002/3 Division Two, then finishing second in the 2004/5 Championship to join the top flight, from which they are now in danger of dropping out.
The point is that there has been no "disease", no one fatal flaw that has seen Liverpool unable to record another league title since 1990, and a wealth of factors to explain why at least one team has finished above them in the league every year since then. One season it would be about Jack Walker's money, another season it would be Arsenal's unbeaten streak, another it would be Jose Mourinho, another it would be Cristiano Ronaldo's goals.
The short answer then, is "it's no one thing (you troll)".