Another reason is that you can laugh at Pascal’s Mugger. b=p−1−pa, and when a (the payoff) is enormous and p (the probability of winning) is tiny, but not as tiny as a is enormous, then b (the fraction of your wealth to bet) is close to p, i.e. almost nothing. The Kelly bet is never a greater fraction than the probability of winning.
Another reason is that you can laugh at Pascal’s Mugger. b=p−1−pa, and when a (the payoff) is enormous and p (the probability of winning) is tiny, but not as tiny as a is enormous, then b (the fraction of your wealth to bet) is close to p, i.e. almost nothing. The Kelly bet is never a greater fraction than the probability of winning.