This decision is mostly due to incompatible drivers and tons of proprietary components on existing devices. Those issues might be partly solved with new chips supporting the Linux 3.3 kernel or later.
Here’s how folks from the open webOS team explain the decision on the official blog of the platform:
webOS started at Palm with the first Palm Pre, and evolved over time to arrive on more smartphones and even tablets. Sadly, it didn’t get the traction HP wanted when it acquired Palm, and it didn’t fit HP’s then CEO Leo Apotheker plans to turn the company into more of a corporate business, and it all ended there for one of the most intriguing operating systems out there.
source: Open webOS blog via Coolsmartphones