Because it lacks a Haptic engine for the user feedback, which perhaps the iPad doesn't incorporate the necessary space . But more importantly, due to the larger display, there could be yield issues to incorporate 3D on a larger scale on an iPad display. Tim Cook also commented when the 6s launched with 3D Touch, it was very difficult to implement on a smaller display, so taken that into consideration, it likely poses a signicant challenge on a manufacturing level.
The next major iPad update isn't likely due to release somewhere in 2019. By that time,
Perhaps it could give Apple the experimenting phase they need to implement 3D Touch on a larger display, if it ever happens.