The history of gadgets would seem to flow from the single-purpose to the generalist. Consider the slow decline of the MP3 player, calculator, pocket watch, and standalone dumbphone: the functionality for all of these and more is easily replaced by any smartphone on the market. The humble pager would seem to be similarly marked for death. A beeper doesn't do much -- it'll let you know somebody's trying to get a hold of you, give you a callback number, and maybe offer a little space for a text message -- and everything that it does do could be replicated by just about any cell phone manufactured in the last ten years.
So why haven't they gone the way of the dodo? Well, it turns out that the group who first used the beeper, doctors, are still big fans, not least because many hospital complexes have poor cell phone reception, because in turn signals from cell towers can interfere with delicate hospital equipment. (Pagers generally receive signals via satellite rather than local cell networks.) Then there are various extra-specialized niches for them, like those pagers you get at restaurants that let you wander away but still get a signal when your table is ready. Sure, your cell phone could do that ... but do you want to give your phone number out every time you go to a restaurant?
As for the other group that famously loves pagers -- drug dealers -- they've long ago switched over to anonymous prepaid cell phones, if The Wire is any indication.