Was going to say bus isn't problem here since there are 3GB modules and they are already used in the new laptops so it is just Nvidia being Nividia.
Well I'm not sure what supply of the 3GB modules is like right now so maybe they're keeping them for the laptops initially.
I agree though it is probably largely product placement, allowing for a 24GB 5080 Ti/SUPER variant later, especially now the 5090 has made space by moving up to 32GB, again dictated by its 512-bit bus.