PalmOS was killed by its design mistakes and technical weaknesses. 16 bits memory limitation, even at the time that was silly. Armlets? Extremely hard to write simple native code or to port apps and games from other platforms. No file system, a custom and rather slow database instead… But they had tons of documents on how to follow their UI/UX guidelines. Basically, they had no real kernel upon which building an OS,…
IMHO Palm was killed by endless M&A activity and other finance shenanigans, which resulted in a lack of long term focus and stability to migrate from the original cramped architecture. And then iphone's multitouch UI drove the final nail.
It turned out easier to use, cloud services (Google apps, iCloud), maps+GPS (G1 had it, iPhone had it soon after), and much better media player features were simply more desirable than enterprise groupware. It also turned out that as cell phone usage normalized, a lot of employers stopped providing cell phones to employees... mostly driven by unlimited talk+text+internet plans... which really favored the non-enterprise-y Androids and iPhones.