They do it because its cheap, traditionally encoders have had more rugged termination/connectors from sealed hardwired cable to MS (military style) connectors.
US digital are an the bottom end of the encoder scale, price wise.
'D' connectors were not really intended for this kind of industrial application, just that they are also a cheap but not industrially rugged.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.