Postulation: All smartphones can accomplish the same general types of tasks, only vary by the feature set (thus complexity) of accomplishing them and the computer literacy of the marketplace. Thus I believe there are TWO market segments of smartphones: Computer literates demanding features, and "Newbies" demanding simple limited experiences.
Theory: As computers permeate the world computer literacy is increasing, thus the market for phone features is increasing. RIM has stayed focused too long on "Newbie" limited simple experiences of email and BlackBerry messaging thus losing customers in mass in todays world.
In order to understand what is happening currently you need to look back to the Pager where it all started. Everyone that needed to be "connected" had a pager before anything else and Motorola was the original key player here.
Motorola (Pagers, StarTac , Razor): Created both hardware and software for their first devices. They were extremely featured limited and very simple to use for the "non-computer literate" crowd of the 80's and 90's and were successful.
As the world passed and the cost of developing software declined they had the wherewithal to pull out of the software market, partner with other companies (J2ME, Android), and focus on their established hardware development segment. While Motorola is no longer a giant in the connected device market, they do still stay relevant ("DROID!") and are still in a great position.
PalmOS (Kyocera, Visor, Treo): Allowed a very small segment of people who were very computer literate in the late 90s, to access Web, Email, Calendar, Tasks list and sync them with their desktop computers. If you were a Realtor you had one of these and loved it because _nothing_ gave you the same functionality in those days.
Their devices were very intimidating to non computer literate people. Palm saw their market as only limited to those comfortable with technology and drove features even further (Windows CE / Mobile hit the scene).
IT did finally occur to them in 2007 the "newbie" market may be a big success (Hello iPhone) and tried to retool their devices and marketing (Centro / PalmPre) but was too late.
Microsoft (Windows CE / Mobile ): Like Palm drove for ever increasingly amount of features trying to appeal to the computer literate, all the while ignoring the "newbie" segment. As with most, have realized that there is money in limited phone experiences and are pouring tons of cash into "dumbing down" their user experience. Unfortunately for them trying to "catch down" to iPhone levels is the opposite of what the current market is starting to want.
RIM (Inter@ctive Pager, Blackberry): Competitors initally to Motorola in the paging markets and they too created hardware and software for their devices. They flourished in the "Newbie" market and rode that wave as long as they could. Unfortunately for them people are driving for more features out of their smartphones and are realizing too late they need to go beyond BBM.
They could make a comeback but only if they do it "Motorola Style" by giving up the Mobile OS market and partnering with Android to bring BES, BBM as an integrated experience with their awesome keyboard based hardware.