Because competition. Not really, but people tend to forget that there is unlikely to be another Wii like phenomenon. Sales will likely drop this gen, but that's due to many consumers moving on. Also, you have to wait to see how many multiple console purchasers come into the picture. Etc
Maybe they should double down on software that uses the gamepad while simultaneously unbundling it. The people who really want gamepad games will buy the gamepad to play them, and for everyone else there's a $150 Wii U that's well into impulse buy territory.
We're talking about console health generally. Keep in mind that PS4 has to carry the weight of two particularly weak second and third place platforms. If PS4 finishes with, let's say, 110M units sold -- very respectable for a generation leading console -- but Xbox One and Wii U sell 50M units total, that will represent substantial contraction of the home console market, while video games in other avenues (PC, iOS, Android, etc.) continue to rapidly expand. Those numbers are chosen semi-arbitrarily, to represent that PS4 doing fine isn't by itself sufficient to show health of home consoles broadly.