you have to remember this was 1998, and apple was the first company going all-in on usb for peripherals, using it to replace the aging apple desktop bus.

the cost concern also has more to do with low end machines in that time frame. if the connector included it, then any devices that used it would need usb support. at a time when usb was still brand new, and most pcs that had it shipped with controllers that only supported a handful of ports.