Reality TV follows its own strange cycle.
Phase I-Introduction
The show is introduced. It slowly gains viewers as it gains word-of-mouth power. Then, usually by the end of the first season, it becomes a huge hit. It is praised as revolutionary, great TV (or crappy TV) critically, and by the next season, everyone awaits the second round.
Phase II-Rise and Fall
Now everyone is talking about the show, and it probably has more viewers than it did the first season. But, the novelty has worn off. People lose interest. At the least, it has faded out of media interest, to be replaced by new, "more revolutionary" shows.
Phase III-Gimmickry
Now, the show is on its last gasp. It is forced to resort to gimmicks to try to bring viewers back. Eventually, it crashes and burns.
Don't believe me? Look at the examples: Who Wants to be a Millionaire. Started in America as a summer experiment. Suddenly, it was popular. Everyone watched it. Then, it was on five days a week, and everyone stopped. Just like that. Celebrity Millionaires couldn't stop the fall.
The Weakest Link followed the same chain.
Survivor is doing the same thing.
Current Stages of Reality Shows
Stage I-Joe Millionaire
Star Search (A revival, but the principle still holds)
Stage II-
American Idol
Fear Factor (Actually somewhere between Stage II and Stage III, note the Playboy special, and now, all-female special)
The Amazing Race
Stage III-
The Mole
Big Brother
Man Vs. Beast (This started in Stage III)
Survivor
Temptation Island (This also started in Stage III)