In the US specifically, because they weren't allowed to operate in the US until November last year. But... they didn't really try to prevent US users from accessing it, just gently pointed them towards using a VPN and paying with cryptocurrencies.

"Coincidentally", they also gave a bullshit advisory position to Trump Jr. and accepted an investment from his firm (1789 Capital), which I'm sure had nothing to do with them getting that licence. But to be "fair and balanced" to Polymarket, Trump Jr. is also Kalshi's "strategic advisor", so there's that!