Read This: In depth on the rise and fall of webOS 58
The tl;dr version: webOS got screwed, time and time again. Sometimes by Palm's own ambitions, sometimes by supposed partners, and sometimes by fate. The Verge's Chris Zeigler has put together an excellent history of webOS as told from the inside, including plenty of behind-the-scenes info that wasn't publicly known before this point. For instance, Verizon was originally supposed to be the launch carrier for the Palm Pre, but backed out, and then a year later with the Palm Pre Plus and Pixi Plus ordered "an extraordinary number" of the devices, only to go with Motorola's Android-powered Droid instead, refusing shipment on hundreds of millions of dollars of Palm inventory.
Then there's the immediate post-HP-acquisition rush and excitement for webOS under HP CEO Mark Hurd, with the Palm group diving headfirst into a brand-new tablet, a new office established to get webOS onto PCs, an actual functioning webOS printer, and more of HP's grand webOS plans. And then all hell breaking loose with a new and disinterested CEO, a disappointing tablet launch, and quick and stunning shuttering of webOS hardware.
It's a sweeping 8000+ word overview of the rise of webOS at Palm and the fall under HP, with plenty of background thoughts to add color to what we all suspected was going on. But know this: the story of webOS isn't over.
Source: The Verge