I think the reason that BB gets more and more boring each time is because most of the houseguests they bring in have seen the game played on TV for so many years they think they have it figured out.
They go in there and say they have watched every season, have studied the game relentlessly, and they are pros at it. What they don't understand is that the cast of people is different each time - every houseguest brings something different to the game and that changes it dramatically.
The main problem that causes is that while these people are playing past games out in their minds, the real game is going on in real life. They can't get past what they know of former shows in order to play the real game, so for the viewers it is the same old same old.......hashed and rehashed!