It's a pretty simple issue, really---North American Spec Defenders were only available for several years. That means the number of road-worthy used vehicles is very small, and hence you don't see many of them in most parts of the country. There are, however, pockets of high Defender density,,, Colorado being one of them. But even that is a fairly small number relative to built-up Jeeps, 'Yotas, etc. Even in a state like CO where you'll see a 1/2 dozen built-up rigs just on the way to work, Defenders are a comparative rarity.
You see more Discos & Rangies because that's what there's more of. Supply & demand keep the Discos & classic Rangies fairly cheap relative to Defenders. In fact a friend of mine just bought a mechanically sound and well-maintained '90 RRC for $2600. The fact that Brits drive on the wrong side of the road
and hence the steering wheel's on the wrong side of the truck
mean that few used Defenders (or Series) are imported or purchased privately for shipment across the pond. Therefore, the total pool of available Defenders is actually shrinking in the US & you are likely to see fewer & fewer over the coming years.
