Are flights expensive in Cleveland? That might discourage a lot of optional travel.
I can believe a lot of people driving to Detroit for flights.
Having other cities within a day or weekend drive is probably big. But remote cities can also have a lot to do in easy range, even if that's mountains or tulip fields in the PNW if you're sick of the other two big cities.
Flights are also driven by corporate presence and residents with a lot of spending money. Cleveland might get less help from either group.
O&D numbers appear to be all over the map on a per-capita basis. Denver is particularly odd, with huge domestic O&D (not connecting) numbers despite not being a Vegas or Orlando. Flying then getting a rental car or bus to ski three hours away is one factor. Being a multi-hub is another, with more destinations and lower fares.
This covers it well.