What is it that makes SMS so expensive to send in the UK? Cost in the US: 0.645-0.75¢ per message Cost in the UK: up to 7+¢ per message to send, inbound 0.9¢

What is it that makes SMS so expensive to send in the UK? Cost in the US: 0.645-0.75¢ per message Cost in the UK: up to 7+¢ per message to send, inbound 0.9¢ I get that there are differences, but a factor of 10 is quite a shift. Twilio is 4¢ too if you want to send from an actual mobile number.