I've thought about it from time to time, I think a lot of what it comes down to is climate and terrain. Australia compared to North America is flat, hot and dry. The need for vehicles with 4WD and high ground clearance isn't as great as in North America where we get significant amounts of snow and there are a lot more hills and mountains overall.
Not to mention Americans have always generally had a "Bigger = Better" mentality so big pickup trucks are more appealing than a mid-size or compact car with the back chopped off and a pickup bed added. Our fuel prices are on average lower as well so why not buy a gas-guzzling full-size pickup? Lol
On the subject of Rancheros and El Caminos, they are super cool but I can't recall ever seeing one with a load in the bed (although I'm only 31, haven't been around all that long). They seem like more of a novelty in this country than a practical vehicle that actually gets used.
