What incentives do sumo wrestlers have to cheat?

Answer 1

They can only cheat by agreement with others. In some cases they agree to let someone beat them in exchange for money. There was a large scandal in the 2000's and the rules are being adjusted to make it more difficult to do so. Because of the openness of the ring and match, with so many spectators, it is pretty much impossible to do anything illegal during the match itself.

Answer 2

Pretty much the same reasons any other sports person has to cheat. It helps them win. Winning means higher payment and better contracts as well as endorsement deals. In some cases they are paid to influence the outcome of the match to benefit betters that have wagered a lot of money on the outcome.