Others have mentioned the improvements in range in more fuel-efficient aircraft. But there's also other incentives at play. Remember when United had a passenger

Others have mentioned the improvements in range in more fuel-efficient aircraft. But there's also other incentives at play. Remember when United had a passenger dragged off the plan on a commuter flight? Turns out that wasn't United, but one of many regional airlines that contracts with United. United's union contract constrains the size of these flights, but the pilots at these airlines make less than members of the union. So there's other incentives for airlines to abandon hub-and-spoke in favor of more commuter flights.