To ensure backwards compatibility and that the bands work with all watch models, the band connector has to be a precise shape with limited tolerances. If they decide to modify the watch's hardware design any further (e.g. making it thinner, more rounded corners, a different size beyond 38/40/42/44), the existing bands might no longer fit in properly.
Also, the current band mechanism is first generation technology when Apple was still new to watch design. Who knows what they might have learned or improved in the 5 years since the watch was released. The current band mechanism isn't without fault either - a lot of people (me included) have had experiences with the bands not locking in, etc so they might come up with a better mechanism.