Both lack of competition and the fact that PI had to have some limits in place to skirt gambling rules in the US. Interesting to see where lines diverge on PI vs like a Euro book for some things.

Definitely often arbitrage plays possible if one is on PI and lives in a jurisdiction where they can bet on politics legally. Though again the small limits and big cuts make a lot of those plays not worth it.

Lack of competition and honestly I think they just have high costs. Payment processors like Stripe and PayPal don't like dealing with this stuff because the fraud+chargeback rates are so high. This leaves you with the expensive niche providers.