Authors really have very little control over where they go touring. The cities are usually selected by the publishers. A lot of it starts with the bookstores. The stores that get a lot of authors make a point of =asking= for them, ordering a large stock of their books, and doing whatever advertising is necessary to make a signing a success. Some bookshops want authors, but aren't willing to sink a penny into ads -- those signings are inevitably a disaster.
And the last tour can affect the next one. If a publisher sends Author X to a particular city or bookshop, and three people show up to buy books (or no one at all, which has also happened), that store is not likely to get a visit from Author Y when he tours six months later.
None of which addresses the particular question of why Oklahoma doesn't get a lot of touring authors. I don't know the answer to that one. The manager at your local bookstore might, however.