The market in the US is much smaller, and as such, the risks are not shared between such a large pool, making it more expensive is the basic information.
In the UK, you don't go abroad without travel insurance. What started principally as medical insurance (since we don't often have medical insurance) has evolved into a specific policy covering a range of eventualities - trip cancellation, theft, baggage loss, legal assistance - for between £80 - £100/year (more if you go to the US, where risk of higher costs are greater). But that type of policy at that price is virtually unknown in the US - we all have it, hardly anyone wants it over there.