* Fees are high. They charge 10% of profits and and additional 5% for withdrawals.
* Markets were illiquid. There aren't enough shares available to put much money to work. There's an $850 limit per bet, but it's hard to reach that amount.
* Bets can take a long time to settle. So even if you are 90% certain of something trading at 50%, you can't put a ton of money into that view, nor can you necessarily get paid quickly.
* Occasionally bets are determined by dumb technicalities. As I recall the bet for Powell being the next Fed chair settled on No, because the date referenced in the bet was a holiday or something, and he was confirmed on the next business day.
If you are smart enough to make significant money on PredictIt aren't you smart enough to make more money doing almost anything else?