I think marketing and a focused message. With the iPad, you know Apple is going to iterate until it gets the experience right. As a consumer and developer, you're willing to invest your time to learn the product or develop applications for it. With HP, you're not sure if it's a novelty item that they'll cut next quarter.
http://www.hp.com/united-states/campaigns/touchsmart/index.h...
caused so much less of a splash than this?
http://www.apple.com/ipad/
It's a given that Apple is better at marketing. But what else? I can envision doing some serious work on the HP devices.