"Sole reason" I can't agree with, but WebOS has bugs that's for sure. I bought an HP Veer around Week 1. Last month I went back to my iPhone4 and I wouldn't go back (even before this announcement).
The Veer lags sometimes, for no obvious reason. Touches often register with the droplet effect, but fail to actually trigger the button you clearly hit. The Palm account had no way to update it online (better not forget to update your CC# before a purchase or you've effectively bricked the app-market). Mail was laggy, and seemed to rarely update properly. I couldn't have both my Work GoogleApps Mail, and my regular GMail on the phone as GMail accounts. I had to setup my personal account as an Exchange account.
None of these were instant deal-breakers, but with no updates, no fixes, and no communication I got tired of waiting for a fix that may never come.
Of course the app-store/market/whatever kinda sucked too.
My biggest peeve was copy & paste. Mentioned in another reply, but before the 2G iPhone I never had a real need for copy & paste since there weren't really worthwhile apps to copy & paste between on the feature-phones back then.
The WebOS copy/paste however absolutely sucks. It's tricky, I never remembered if it's Option+C or gesture-area+C no matter how many times I did it.
Oh, and the Veer screen was OK for most stuff, but waaay too small for web-browsing without Android's ability to re-flow text on zoom.
Signed, -A Guy With A Veer Sitting On The Shelf