Yeah, money and priorities. In Minnesota (2002 average bid prices - closest I have to hand) the solid plate beam guard rail runs about $10 per foot and 3-cable guard rail is about $7 per foot. Add in periodic end treatments and design costs and it is probably $40,000 - $50,000 per mile per side. (It may be higher in mountain areas due to the costs of getting material to the site, difficulty of working in the area, etc.) How many miles do you cover? It’s not a nice answer, but decision makers have to look at where there is the likelihood of a higher concentration of accidents and concentrate spending on those areas.
I don’t know about Colorado or Arizona, but in Minnesota the gas tax hasn’t increased since 1988. That means a lot of things that should happen, don’t.
Guard rails make snow removal a bit more difficult, but not a lot more. (No cite available, but I was a county engineer for a while and know from supervisory experience.) In plowing rural areas without guard rails, snow plow operators can easily wing back (push out with the side, or wing, plow) the snow to clear shoulders or push the snow back away from the road to minimize drifting. In areas of guard rail or other obstacles they need to slow up or stop, bring in the wing plow and advance using the front plow to push out the snow. Often times follow up clean up is necessary around guardrails. This involves loading the snow that is left next to the guard rail onto trucks to haul away or using a large snow blower to blow it to the other side of the rail.
All in all it is not that difficult though and I would rate snow plow maintenance as not being a consideration in whether or not to install guard rail.