The real story is more complicated, and covered very well in a book called "Losing the Signal", written by journalists at the Globe & Mail.
RIM had an engineering-first culture that was attuned to the needs of enterprise customers and wireless carriers first and foremost. Those customers prioritized email delivery over bandwidth-constrained mobile networks. RIM made big money off of Blackberry Enterprise Services (BES), which included dedicated servers to handle push email and compress web traffic for improved speed.
When the engineer CEO Mike Lazaridis saw the iPhone, he thought it'd fail because AT&T's 2G and 3G networks would collapse trying to load desktop websites and online video.
Which they did. The biggest complaint about the early iPhone in the US was that it dropped calls, and that websites loaded slowly (this was before responsive websites, srcset, CDN caching etc were common).
Ultimately it didn't matter, because consumers and businesspeople wanted the iPhone and accepted the tradeoffs.