What happened?
Apple came in with better expensive hardware and a better OS
Google came in with a better (free) OS
Many other companies came out with better low-price hardware
The fastest messaging server (BES) became free for some reason... (couldn't compete with free* Activesync?)
Social networks and other messaging platforms overtook the free email alternative (BIS) and mobile-to-mobile messaging (BBM)
The most secure encryption fell by the wayside -- it was so secure that it was a liability, as some governments refused to let BES operate in their country because they couldn't spy through it.
The network was another liability, as people had a $10/mo higher phone bill to use a Blackberry than any other smartphone.
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Blackberry still have the fast network, secure encryption, and a number of other assets, but as yet have failed combine those assets into any sort of compelling product.