Possibly because they have retransmission agreements with the Tucson stations and not enough with the LV station to make it worthwhile yet?
I highly doubt it.
DirecTV has HD locals and retransmit agreements for all big four networks in Las Vegas.
Las Vegas
3 KVBC (NBC) - Sunbelt - no HD retransmit history with Dish
5 KVVU (Fox) - Meridith - good HD retransmit history with Dish
8 KLAS (CBS) - Landmark - good HD retransmit history with Dish
13 KTNV (ABC) - Journal - no HD retransmit history with Dish
Tucson
4 KVOA (NBC) - Evening Post - no HD retransmit history with Dish
9 KGUN (ABC) - Journal - no HD retransmit history with Dish
11 KSMB (Fox) - Belo - good HD retransmit history with Dish
13 KOLD (CBS) - Raycom - HD retransmit Dispute with Dish in Honolulu