[zamadatix]

The short response would be the "web browser" started to fill the role of "cross platform on demand app sandbox" instead of "plain hypertext document viewer" and that will forever be evolving to match the capabilities of devices themselves. More than any other contributing reason to the churn (including the highly charged/passionate reasons), if you defined a "web browser" to be a limited subset of the functionality the user desire for a constantly evolving then an "on demand app sandbox" app with a different name would still exist and it'd just be everyone using that application instead of a web browser.