A remote code execution bug in ios is valuable - it may take a long time to detect exploitation (potentially years if used carefully), and even after being discovered there is a long tail of devices that take time to update (although less so than on android, or linux run on embedded devices that can’t be updated) That’s why it’s worth millions on the black market and apple will pay you $2 million dollars for it

An XSS is much harder to exploit quietly (the server can log everything), and can be closed immediately 100% with no long tail. At the push of an update the vulnerability is now worth zero. Someone paying to purchase an XSS is probably intending to use it once (with a large blast radius) and get as much as they can from it in the time until it is closed (hours? maybe days?)