Several possible reasons.
1.Cost. Until the builder puts them on a rig add in the cost of the rims in addition to the tires if an owner wants to make the switch.
2. Availability. There might be some concern on wide spread availability beyond the interstate corridors should you have a failure.
3. Consumer demand. The builders probably won't add them as anything more than an option until we the buyers start demanding them.
4. Fear of the unknown. Duals have worked pretty well for a very long time. Why switch?