The most money can be made in markets which have the worst correlation between reality and the market price. There are types of markets on PM which are very far from reality. There are types of markets which are very close to reality. It isn't surprising that sports markets are in the first category.

There are also political markets, where it is clear campaigns manipulate the prices for the same reasons they will publish polls showing they are gaining or ahead. The CA governor market is especially far from reality. This is compounded by the fact that Americans can't trade this market so the distance from reality is especially bad for American election markets.

That being said, that most people don't make money on PM isn't surprising. The same thing is true for most markets. You only invest (business) time into something if you are getting a return. So those that are actually good at making predictions put the time into making trades, so they are the bulk of the trades and the bulk of the profit. Same thing happens with day traders on the equity markets.