The game is still in-development, no? We don't know how well the PS4/bone versions were running as of the decision to drop last-gen...
Even if the overall art and scope were designed with las-gen in mind, they still might have pushed the content a bit too far and a lot of work was still needed to make those versions shippable.
Not having to optmise the shit out of those versions might free them up to polish up the new-gen ones and ultimately allow them to release 2 good versions of the game rather than 4 mediocre ones. They may be avoiding the same mistake CDPR made with Cyberpunk...