They never learn
This is like watching a parade of knights setting off to slay the dragon, one after another, and never returning.
I have a Nokia N800, one of Nokia's early attempts at an internet tablet. It could have gone somewhere (and they did release a few subsequent models and a phone based on it), but they failed to support it and abandoned both the devices and the Maemo O/S.
I'm listening to music from my HP TouchPad as I type this (running Pandora with the very nice Apollo app). It's been one of my most-used devices over the last year. I'd never have bought it in the first place if not for the firesale price, but now that I've used it, I'd buy another one. And I've spent freely on apps for it.
Until one of these companies gets some management with the conviction to suck it up and make a true commitment to an uncertain market like Apple did, they are doomed to be gradually fading also-rans.