since the money started pouring in, it's become one of those catch 22/vicious circle things whereby whoever was at the top when this happened has stayed there.
They get more prize money, they get more TV money, they get the money from the champions league, gate receipts and merchandising are up, plus being in the champions league enables them to attract better players, plus they can afford higher wages, so the strong get stronger and the gap increases.
Also if they're a "glamour club" with a global reach, the football authorities in their country might not want to see this changed. Then Premier League chair Richard Scudamore once said that Man United being up there every season was actually good for the league as it meant a bigger audience and more world interest, comparing it to more people tuning in when Tiger Woods was about to win a tournament than they would if it was a lesser known golfer (I'm guessing these comments were pre 2010!)