Likely the technology was not ready or the internal space was not available for older internal components and bigger batteries consuming space. Now, the iPads have reduced components and smaller, more efficient batteries, where perhaps a Haptic motor could be possible to fit in the iPad . Then incorporate the display for 3D Touch. I think 3D Touch will be here for the iPad eventually, but Tim Cook commented on the difficulty with the iPhone with 3D Touch.
Amazing technology that can fit into a device that's only 7.5 MM in depth.