Very simple answer to the question
The reason is because we the fans accept mediocrity
We have very very low standards
Even if I accepted your premise that we the fans accept mediocrity, and I don't (for every unquestioning clod who thinks our list is just fine, there are hundreds who know it's not), how does what the fans think have any bearing on how the club functions anyway? What can we as observers of the game and nothing more do to tell our clubs management that we demand better? Dump chicken poo on the doorstep? Tried that. At the end of the day our only recourse to protest is to cancel our membership,which is like cutting off your nose to spite your face anyway.
The problem is that the fans are too loyal, this breeds complacency in the board and footy department which leads to crappy results.
If our membership dropped , say 10000 members next year, maybe the board will pull their finger out and actually sign a sponsor, and get a coach with half a brain.
As it stands, due to the members loyalty, and our low standards in accepting mediocre results, the board does nothing and 7 years on we are stuck with a proven failure of a coach for another 2 years.
This is directly the members fault due to accepting the rubbish we have been given.
My advise, don't buy a membership. Make the club struggle, then maybe they will be forced to act with urgency.