It is easier for the SEC to investigate futures, stocks & options. Some of the prediction/gambling markets use crypto and do not participate in "Know Your Customer" regulations.

You’d think so, but these types of people are afraid of stocks or options and find prediction websites easier to use.

Honestly not that big a difference here. Both can be loosely interpreted as gambling, especially with the same rough upsides (money without production or consumption!) downsides (addiction, critical existence failure of all your savings) and the corruption and perverse incentives they invite.