Apparently this "Why can't I have DSL?" question is a common complaint. I'm one of those people in a duplex whose neighbor gets Verizon DSL, but Verizon is saying I can't have it. The DSL line enters the house on our side and runs through my basement before it gets to the neighbors. Doesn't that mean we're "closer" to the Central Office than our neighbors, at least as far as "copper wire miles?"
We figured out the DSL lines ran through our unit when our neighbors' DSL needed to be serviced and the technician had to come into our apartment to check out the lines.
This is maddening and is costing me an additional $40/month for internet.