Because the outcome of a sportsmans type / charity bet, should be more or less in the ballpark of neutral on both sides. Eg if I said to you that the next election in 4 years time will be won by the Conservatives and you thought Labour, then despite one being a little shorter than the other, there are lots of variables and it could easily be a charity bet. In this instance he could put that same £50 bet on with a bookie and not v you and receive £1,300 if he won and donate £50 to the charity of his choice from it, rather than having the bet with you and having risked the same outlay, you pay £50 to his charity and he gets nothing.
*edit - if Reform win that election, I suggest you both keep your £50 as you'll need it!