It is mainly due to shipping costs! If I am able to buy from Amazon UK, they charge more than the item's worth to ship to the USA :-( I thankfully have a mate in the UK that I can send items to but I (try to) do that sparingly as to not make him go to the post for me so often!
Shipping from a singer's web store is usually half of the item's cost/worth so I have been trying to purchase items that way.
To your point, yes, I don't understand why a fair amount of items are strictly UK ONLY! :-/ :-/
This is absolutely ridiculous. She's had the time to sign what must've been about a billion artcards for this album (they were on sale forever in the US store, and they had a huge sale to clear them out of the UK store), and now an additional 1000 UK Exclusive posters, and yet she still won't sign the vinyls.
Thirteen months is a ridiculous amount of time to wait for any preorder.