They didn't have a "glass slab" style phone until literally every other phone had been like that for years and years. They were also late to the game with cameras, IMUs, and other hardware features that we've all come to take for granted.
My guess: They listened too much to their enterprise customers. Enterprise customers have very, very specific needs that are essentially the opposite of "general purpose" that would translate into useful features for anyone else.
They didn't even try to innovate!
What the market was saying in 2008 was it didn't actually give a shit about BES, didn't give a shit about BBM, software keyboards were fine if they didn't suck, and real browsers on phones were important.
Instead of really listening to the market RIM just kept making the same devices as before. The iPhone and even early Android phones blew the doors off BlackBerries. By the time RIM decided to join the rest of the world it was too late. They were releasing devices in 2010 that couldn't compete with iPhones and Androids from 2008. The BlackBerries were up against the iPhone 4 by 2010, they were a joke in comparison.
I went from a BlackBerry Pearl to the first iPhone. Even in the admittedly early state of iOS 1.0 it was vastly superior to the Pearl. It wasn't even funny how much better of a device it was. Every iPhone after that just increased that gap over whatever RIM was selling.
Blackberry phones were sort of similar, but as enterprise-first devices they always had a reputation of being mostly a work email device, not personal. They still had filesystem access and microSD expandability though.