Not really just one thing. Pent-up demand + typical summer seasonal demand + capacity reductions. ATL-BMI went from 3xCR9 pre-COVID to 1xCR9+1xCR7 post-Covid. With only 2x flights per day, this greatly limits connecting options and there's a greater chance of married segment availability limiting booking options to higher fare classes. There's a decent chance there will be some cheaper options if you book LAS-ATL and ATL-BMI flights on separate fares than the price on a single fare (it will also open more connecting options as you will not be subject to 4 hour connection limit when booked on a single fare).