They take up less space than a full size bike and don't fold. Folders do require occasional maintenance to their hinges and pivots. Those hinges also add weight and cost,and can cause flex in the frame.
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C'dale BBU('05 and '09)/Super Six/Hooligan8and 3,Kona Dew Deluxe,Novara Buzz/Safari,Surly Big Dummy,Marin Pt Reyes,Giant Defy 1,Schwinn DBX SuperSport,Brompton S6L/S2E-X/M6L-X/S12 T Line