They could but they would have a lot of developers sniffing around the edges trying to figure out if HP was really serious this time. Last time I went to their developer conference they said they were really serious, and then 3 months later dumped the entire thing.

Unlike Google, they actually had a decent tablet OS. They had ceded the smartphone market to Apple and Google. But at the time the only real tablets were coming from Apple. Some idiot bean count of a CEO saw the competition (iOS and Honeycomb) and got scared.

Its gone. HP lost its credibility with app developers - they've moved on and they're not coming back.