It's simple, secure, stable, allows for group chat, notifies you if the message was received or not, and is pre-installed on BlackBerries so everyone with a BlackBerry has it.
I was reading the page looking for grandparent's question, and I still don't see why bbm would be such a huge feature that bb owners just can't live without. Sure, I can see how it may beat sms where international communication may be concerned, and texting someone in a different country doesn't work, and I can see how the group chat is useful, but every single other IM client can offer the same thing... I just don't get it.
Ted Livingston worked for RIM and saw problem, a big gap that needed to be filled in. Which was, taking the best black berry app and making it accessible on every major smart phone out there.