Ah, that makes sense. Thanks.

I think one issue that is more unique to the west is that the distances between towns are generally greater.

Quite so.  I can't think of anywhere east of the Mississippi River where there's a 90 mile gap between population centers but there are at least two such places on the western ACA routes.

It was in part the wide open spaces of north central Wyoming that convinced me to stop in Cody; I just couldn't suss out where I was going to be able to find water to drink after I'd exhausted what I started with.  

My supply - a two liter Camelbak, two 24 ounce bottles, and two 20 ounce bottles - was good for about 40 miles of riding if I was diligent about staying hydrated: something I was indeed trying to be conscientious about- so the 60+ mile ride from Cody to Ten Sleep and the 70+ miles from Clearmont to Buffalo in particular had me concerned.

Given those concerns plus the lack of obvious places to turn to if I encountered any sort of problem persuaded me that for myself at least the game wasn't worth the candle.  Maybe I psyched myself out- after all, many riders make it across those spaces every year- but my risk tolerance wasn't up for it.

My supply - a two liter Camelbak, two 24 ounce bottles, and two 20 ounce bottles - was good for about 40 miles of riding if I was diligent about staying hydrated

Wow. That's a LOT of water. I'm certain I could not force myself to drink that much water over the course of 40 miles. Of course, my urine looks like Tang (that orange stuff the astronauts drank) when I get back from a 100-mile ride. So I'm probably in a constant state of dehydration.

I always carry a Sawyer Mini water filter when I'm touring. I rarely need it, but when I do, I'm very glad I have it. The day I rode from Basin, Wyoming, through Ten Sleep, and then over the pass to Buffalo (110 miles), I filtered water from a spring while I climbed up to the top. 

The last thing my primary care doctor told me before I left was "Don't get dehydrated": advice which I took to heart.

At the rate I was traveling, 40 miles would take between four and five hours.  I sweat pretty freely in most circumstances, but in the semi-arid west it did what it was supposed to do and kept me cool(ish) through evaporation.

I was carrying a water filter but it seemed nearly every stream and river was separated from the road by either unmanageably steep slopes and / or barbed wire fences with "No Trespassing" signs prominently displayed.  I had no desire to test the patience and forbearance of the landowners (who I assumed were probably armed) by crossing their marked boundaries.