Exactly what you said. The fuel economy on diesel engines is significantly better than gasoline, because the lack of combustion keeps the engine cooler, making it more efficient. As Ford/Dodge/Chevy owners can attest, they also run a hell of a lot longer, and the extra power doesn't hurt.
Same reason that most of Europe uses diesel, its just an all around better fuel type to use, for any platform.
Diesels used to be a great alternative to gas. They were about 40% more efficient, had fewer moving parts, were easier to work on, and definitely lasted longer. However, with the current advancements in gasoline tech, and the high cost of emissions equipment, new high maintenance schedules, and the way over the top price gouging of diesel fuel, there is no way that a small diesel can affordably compete with a direct injected modern gas motor. (Unless you absolutely need the torque, like in HD platforms.) IF Toyota were to put a small (3.5-5.0) diesel in a roughly 5,500lb truck, it would make about 300 HP and about 550 lbs of torque. And probably get about 25-28 MPG HWY at best. (Which is great, although Creo would want more torque at any cost.) But the cost of the damn thing would still be, say conservatively and by some magic, around 5 or 6 thousand dollars more than the 5.7. It makes no sense to pay five grand (which is about 500% more than the cost to upgrade to the 5.7 from the 4.6) when you only get 40% more efficient, but pay about 118% (18% more) of the price of gas. Add in the cost to maintain the damn thing, and it makes even less sense for Toyota to try and risk putting that much on already price and maintenance conscious buyers. The only one ballsy enough to do it is Ram, and I fully believe they will be the first. But then Ford will rock their world with marketing and a much stronger buyer segment. Once these guys prove out the market, GM will jump in, and then Toyota. Toyota will be last, but of course, they will have the most advanced tech, but they seriously need to gain (probably double) their market share in trucks for it to even be worth thinking about.