I think they are popular because for some weird reason people thought the BB Messenger was free with the phone. They didn't realize that you still need data service. And if you have data service there are all kind of IM that you can use, besides BBM. Still you ask some people who own them why they still use them, and they say that it is because they can't afford to pay for data services to use whatsapp, and BBM is free.
BB are not cheaper. If anything, they are more expensive. In Claro or Orange a regular BB, one of those that everybody owns, costs 12K DOP, which is more than almost any Android. A Z10 costs the same as a Galaxy S4 or iPhone 5. They're not cheaper to maintain either, they break every 3 months on average, and have all kind of software issues that cause the phone to crash and need flashing regularly. Yeah, you can find some cheap used ones around, but they are normally very old models that don't work well (heck, the new ones don't work well).
However, progress cannot be stopped. And even the least knowledgeable barrio girls (BB's last stronghold) don't ask for BB anymore, now they just say "papi, regalame un galasi"