It's a simply dynamic, really. When the server first comes online for a new event the rates are good to encourage participation. But eventually the rates have to return to the statistical trough for the math to work, so during the following week(s) it gets harder to succeed. Most people at that point are still vested enough that they continue to participate, even though not nearly as many people are winning good prizes...so it's a win for NCSoft.
As for the players, meh, you knew what you were getting into when you downloaded the game. I mean it's not a coincidence that one of the chief characters in game is named "Mammon." If you don't know what that word means, google it.