A US tour for a UK band is very expensive.
you can't use your own gear, the voltages and CPS don't match.
so you have to rent the gear, and rent the tour vehicles and equipment,
book the venues, book your rooms, find a crew,
put the advertising together across the country, and the different paper and magazines for ads, press releases etc,
most people think that bands get rich on tours,
not the case,
It's a real nightmare if you haven't done it before.
and VERY expensive to put together.
It takes a lot of good people that have been doing it for years to pull it off, and they all have to be paid for their work too. They don't like to risk their own money very often.
a lot of bands go on tour, and come home owing more than they made. I have seen this happen.
It's especially hard for a foriegn group to tour, when no one here know who you are.
there are already thousands of good American bands you will be competing with, that at least have some regional fame.
Until you can prove that you can bring people into the stadium or club, no one will want to risk the big money on you