At the time the Zeiss Ikon was under development, I believe that Kyocera still held the rights to the Contax name, even though there were no new products on the market.
Next, Zeiss doesn't have camera manufacturing facilities. To build cameras is no easy task. In addition to manufacturing space, there also is the investment in equipment, training, additional employees, raw materials, etc.
That's a hefty investment.
It made great sense to go to another company that already had all of this in place, including a track record in making rangefinders.
Remember, Zeiss Ikon made cameras but not lenses. Carl Zeiss made lenses but not cameras. And although the new camera is branded Zeiss, Zeiss still doesn't actually make the camera.