In order to use some of the functionality of the health app, you would need to walk around and be active while carrying your iPad. I'll guess Apple doesn't see that happening.
They could provide a modified version that would cover some functions.
I did see this while Googling the question. So things may change.
Apple's Health app leverages the iPhone's embedded motion sensing chip to passively track steps -- a chip iPads don't have. HealthKit can also integrate data from select Bluetooth LE-enabled heart monitors directly, while most other devices need to feed their collected data into HealthKit via their app.
iPads aren't going to be the leading form factor choice for feeding real-time heart rate data from a wearable, and Apple's decision to not include a passive activity tracking feature on the iPad is an understandable one. Apple's Health app, however, is mostly a dashboard that displays a variety of collected health data points and associated trend lines. That sort of user experience seems like an obvious fit for the iPad's larger screen.
The iPad could have also been a HealthKit access point for Android smartphone-toting patients at healthcare systems integrating their patient portals with Apple's Health.