Concur is a booking platform front-end to any number of GDSes and travel agencies, and simply implements whatever travel policies your corporate travel management chooses to implement. Some companies explicitly exclude non-flexible fares like DL's Basic Economy from Concur - they don't even show up.
Your beef is not with Concur, it's with the policies your travel managers choose to implement in Concur.
P.S. Welcome to FlyerTalk!