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Same reason car manufacturers use custom plastic connectors, like for instance in personal experience, a VW Passat wagon electric radiator fan connector. Connector failed, approx. $200 motor/cable assembly junked and replaced. If they had used a Cannon plug (maybe max. $10 in bulk) it would outlast the car - that's why military and aircraft electronics use them or the equivalent from Amphenol, etc.
If you want a cheap source for 4 and/or 6 wire stepper motors, check out used/trashed printers. I got a great husky 6 wire one from an OLD Epson printer. Nice toothed belts in those, too, btw.