When the 360 and PS3 launched, you had two options: release your game on PS2 etc, where it had to be SD and deal with less power, but have larger base, or release on PS3 etc, where you had that power, but less units.The Wii U's first two months sales were equivilant to the amount the 360 and PS3 shipped during their first two months. Why do people keep on saying that it is doing badly? It's doing good, not amazing, but not bad by any sense of the word
With the Wii U, the options are release your HD, heavy requirement game on the PS3 and 360, with a large player base, or the exact same power on Wii U with no units out there.
Think it's clear why companies aren't developing for it. And if those numbers don't increase, and fast, companies will just move to next-gen without so much as a second glance at the Wii U. That's why them selling the same numbers is not good enough.