Poker
players love online tournaments because there are chances to earn big amounts with relative low investment.

The spectators love to see these players battling it out, because the game is more based on skill.. and not so much on luck.
Also, Online poker tournament allows its players to compete for huge money from the comfort of their homes and the spectators are also watching them via live streaming from their homes.

Watching someone battling it out against a Slot machine can become boring very quickly....

Do you know how long the tournaments with low buy-in and high payouts last? Late registration is 12h sometimes... and after that, you can add min +3h (depending on the number of players/if there are available re-buys/etc) before the end! So count it around 15h, a crazy working day!
That's why Bitcointalk poker games were great! Without late registration and re-buys... approximately 3h per game! And when you add sponsored pool it was a low buy-in tournament with possible high rewards! It's hard to find something similar around... to not say impossible!