[tantalor]

Why wouldn't it be

[Eisenstein]

It creates financial incentives for anti-social behavior and the proposed benefits of it being a way to determine truth are dubious and self-serving and would not be worth the societal cost regardless.

[dymk]

https://en.wikipedia.org/wiki/Insider_trading

[sethops1]

Because we have laws that regulate gambling. "Prediction markets" get around them by bending the definition of the word "gambling".