Dual SIM phones don't have the fastest bandwith (e.g. not LTE) and only handle ATT and TMobile. TMobile generally roams on ATT where it doesn't have coverage and prepaids usually don't support roaming on small regional carriers. There isn't a lot of incentive.

You could justify it for border commuters but that is a small pool. On the US/Canadian border the cheap Canadian carriers don't have great coverage except in Metropolitan areas. If I were a Canadian, I'd be more tempted. A Mexico/US commuter would have a lot of reason for a dual SIM phone, but to make either approach work you'd probably need a virtual number on top of your real numbers redirecting the calls to the appropriate SIM.

I have ATT unlimited global data so my iPhone covers my data needs. When I'm travelling I carry a small Nokia Asha for voice calls with either a Toggle Mobile, Piranha, or local SIM.