Well, I'm building one...
I think primarily, despite what so many of the UAW raised people around here want to admit, they are tough trucks. Easily the best small truck of any manufacturer..
Looking at whats out there, primarily, you had Toyotas, Fords, and Chevys.... yeah, there were Mistubishi might-maxs/Dodge D100s, Dodge dakotas..but those 3 are the big 3(including the Isuzu Hombre{S10} and Mazda B-series{Ford Ranger})..
The Chevy S-10, well, never known for reliability was it? Early 80s models rusted out, had a crappy 2.8 V6, reliability issues with the 700R4s, etc..
Ford rangers with the TTB front end made decent wheelers, as can Bronco IIs..
Early Toyotas had the HUGE advantage of having front solid axles, something nothing else but the Jeep Comanche offered. So there are lots of parts out there. I have two solid-axle toyotas myself. The axles are strong(roughly equivelent to Dana 44s) with tons of aftermarket support. Its easy and cheap to swap these into later trucks.
In general, Mechanical stuff has a lot to do with it. Toyotas are stout. Toyotas have been used all over the world in the worst conditions, like Rovers and early Jeeps. The 22R series of engine is extremely reliable, I have 401,000 miles on one of mine, with nothing but routine maintenance. The transfer cases in most are strong gear driven cases.
In terms of aftermarket, there is a much better aftermarket than any domestic truck. So parts are CHEAP. The reduction unit in the gear driven transfer cases is bolt-on, so for less than $300(plus a donor T-case), you can do a Marlin Crawler dual low range setup. High steer can be done for a couple hundred including DOM rods.
Lots of trucks out there, so theres lots of parts to be had.
Strong, reliable, great aftermarket, etc..
Here's mine, hopefully its driving next week:
