All different factors can alter the Pay-out and Prize structure, which have already been mentioned, Number of expected players, Blind structure, Rebuys, add-ons, and freeze outs, The prize offered by the site has to be paid out to players who make the money Regardless of how many join the Tournament, So if they feel it is better to reduce the buy-in to attract more players, they can but they cannot reduce the advertised Pay-out, because they will be losing money, It's their loss!
Most games I have entered into in a Casino only stipulate the Buy-in The Prize money being determined once all players have been seated, and the Add-ons and Rebuy periods have ended, because they are limited on how much money they can rake from a game, be that 5% or 10% or whatever so even if they have less players than they expect, they can still lose money because they have to pay out dealers and floor staff wages.
Running a Casino
poker game is not as easy as players think, and I am sure many
casinos would prefer not to run Tournaments if they could, but Poker is probably the most popular game in the World and if it gets people into the Casino, It is the other games that make them more money like
roulette,
blackjack and Slot machines. Even Cash games make them more money because they can take a rake from virtually every hand played, So Tournaments might get more people into a Casino but they do not always make them more profit.