The reason that Condor appears in the OTA and in the GDS is that Condor have agreements with these companies, but MT do not.
Historically, DE was part of the Lufthansa group and often used where LH couldn't be for political reasons. DE operated to Taiwan in place of LH, for example. DE has infrastructure closer to a scheduled airline, but MT has grown out of the UK charter industry and has never sold via GDS's as far as I know.