I would list the following as the main reasons
Exchange rate. At $2.10 per £ it is way out of line with costs. Its driven by the low interest rates on the US which are there to stimulate the economy. (Brits flying in the US are just one part of the economy).
Fuel taxes are lower
VAT here is 17.5%. Sales tax in the US is typically 0 to 8.5% depending on the state.
US airports are partly funded by fuel tax. Over here Gordon just stuffs the tax in his packet and then reminds you that you have to pay VAT on the landing fee! Our government puts nothing into GA.
Utilisation is generally higher in the US as the weather is better in the southern half.
FAA rules for maintenance are much less costly than over here. The CAA/EASA think that policing everything (and charging for it) makes GA safer. The accident statistics would indicate otherwise.
But I think the fundamental reason is that GA in the US is a right, in the UK/Europe its considered to be a privilege.