B-23 HOW DID MR. SATAN WIN THE TENKAICHI BUDOKAI?

In the first one that he won, none of the competition had strength even close to that of Chichi's.

In the one that Gohan, Goku, Vegeta, Kuririn, Piccolo, and #18 entered, Gohan's ki was sucked by Babidi's pawns, while the rest mentioned above, with the exception of #18, left the tournament to help Kaioushin (Piccolo forfeited the fight to Kaioushin). Trunks and Goten were disqualified, and all the other opponents were knocked out by #18 and Trunks/Goten (or lost in an earlier match). #18 offered to throw the match if Mr. Satan would pay her 200,000,000 zeni (the typical monetary units used in the Dragon World).

After the Buu storyline, the Champion of the Budokai is the last guy the winning contestant get to fight, and since no one could defeat Mr. Buu (Goku and the others didn't join the tournament), it would always be Mr. Buu against Mr. Satan, and Mr. Buu would always throw the match (being the naive good friend of Mr. Satan).