The latter two all Essential Air Service cities, and Decatur probably is as well (although it's close enough to Champaign and Springfield that I can't see why). Air Midwest serves a number of plains EAS cities out of Kansas City, and they'll use B1900 planes to do so. EAS flights are federally subsidized, so load factor is more of a bonus than anything.

Quincy and Marion had been served by American Eagle (I forget which actual operating company) out of STL recently. Years ago, United used to serve Quincy.

The interesting question to me is with which carrier (if any) will Air Midwest code-share on these flights. On the KC routes, they have codeshared for at least 15 years with US Airways, and for a number of years with Midwest Express - but neither of those is really an option at Midway. Before their implosion, ATA would have been the likely candidate, as Chicago Express flew turboprop commuter service for them.