Look at the geography of the region. There are no schools in WA, OR, CA, or AZ that are really logical choices for membership. In the states due east of the region, despite tremendous growth, there a lack of density and population in place to put their programs in a league with the west coast. That's a lot of open space between the coast states and the bulk of the US population, states with little connection with the Pacific region. So why expand?
Actually, lets look at historical state population trends in the west, and pay close attention to the state populations of Arizona, Oregon, and Washington and when they entered the Pac 10, and how close Utah and Nevada are currently to the populations of those states when those current Pac 10 schools entered the Pac 10:
Utah: 1950 = 688,862; 1960 = 890,627; 1970 = 1,059,273; 1980 = 1,461,037; 1990 = 1,722,850; 2000 = 2,233,169; 2003 Estimate = 2,351,467
Arizona: 1950 = 749,587; 1960 = 1,302,161; 1970 = 1,770,900; 1980 = 2,718,215;
1990 = 3,665,228; 2000 = 5,130,632; 2003 Estimate = 5,580,811
Nevada: 1950 = 160,083; 1960 = 285,278; 1970 = 488,738; 1980 = 800,493; 1990 = 1,201,833; 2000 = 1,998,257; 2003 Estimate = 2,241,154
Idaho: 1950 = 588,637; 1960 = 667,191; 1970 712,567; 1980 = 943,935; 1990 = 1,006,744; 2000 = 1,293,953; 2003 Estimate = 1,366,332
Oregon: 1950 = 1,521,341; 1960 = 1,768,687
; 1970 = 2,091,385; 1980 = 2,633,105; 1990 = 2,842,321; 2000 = 3,421,399; 2003 Estimate = 3,559,811
Washington: 1950 = 2,378,963
; 1960 = 2,853,214; 1970 = 3,409,169; 1980 = 4,132,156; 1990 = 4,866,692; 2000 = 5,894,121; 2003 Estimate = 6,131,445
Do you notice something here. In 1979, both Arizona and Arizona State both joined the Pac 10. In 1980, the closest census year, the population of Arizona was 2,718,215. As of 2003, Utah has a population of 2,351,457 and Nevada's population is 2,241,154. SLC and Las Vegas now are very similar size metro areas to that of Phoenix then. Getting very very very close to what the population of Arizona was when ASU and U of A entered the Pac 10 from, what was that conference called then, oh yeah, the Western Athletic Conference (formerly aligned with BYU, Utah, CSU, Wyoming, UNM, etc).
Look at Oregon. During the last century, I believe Oregon and Oregon State were aligned with Pac 10 schools for awhile, then were brief members of the WAC, then in the late 1950's/early 1960's became aligned again with the Pac 10. In 1960, the closest census year, Oregon had a population of 1,768,867, which is currently smaller than both Utah and Nevada's population as I have listed.
Washington (U W) has always been aligned with the Pac 10 schools. I believe WSU was aligned with the Oregon schools for awhile with the WAC very briefly. I could be wrong on this. But notice in 1950, the state of Washington's population of 2,378,963 is very similar to the current populations of Utah and Nevada.
I still believe that the only way that the Pac 10 expands to 12 is if the Big 10 does it, and they need a championship game to stay competitive. That may be along ways away. Some think it will never happen. I would never say never. But as far as the arguement of Utah and Nevada not having enough population, or are too sparse, I don't think that holds any water, based on the observation above. Both Utah and Nevada have very comparable populations to Arizona and Oregon when those schools in those states either joined or rejoined the Pac 10. The Pac 10 may still prefer CU and UT to UU and BYU, but I still say that UU and BYU are the most likely 3rd and 4th best choices for the Pac 10 if they do indeed ever decide to expand. Sparse population of Utah and Nevada and even Idaho in time (the most nearest states to the Pac-10 footprint) have nothing to do with it IMO.