They know that Arashi has tons of fans around the world from sales, votes from Mannequin 5, and choosing the Arafes songs, so why not?
Then I considered the cost to profit ratio of holding a concert in the States and economically it's really not worth it.
For a full Arashi concert experience they would need a ton of staff, mechanisms (moving stages & such), effects (fireworks, waterworks, etc), and a huge enough venue to hold it in.
I know Arashi has a lot of fans in the States but it would probably only be enough to hold a concert for two nights and not even to a full audience.
The costs would be too much and the profits too little to hold a concert anywhere outside of Asia.
The only other option would be for them to hold a lower-key type of concert that wouldn't cost as much.
Honestly though, I would rather go to Japan and experience the full Arashi concert experience rather than them come here and do that.