Once upon a time there weren't any RC Short Course trucks, there were just stadium trucks, which are open wheel and not very much like trophy trucks or short course trucks.
Tamiya did have the TA-02T-based #58161 "4x4 Racing Truck F-150" released in 1995 that I coveted in the Tower Hobbies catalog as a kid. There was also a Toyota Prerunner (#28136) and Chevy S-10. I was busy with life at the time but it got a re-release as #58495 Ford F-150 1995 Baja Version in 2011. They released it with a generic "toyota" truck body as the Desert Fielder #58537 that same year, probably due to licensing issues.
Those Tamiyas were cool but not enough to start a racing class in the US, which happened in 2008 or soon thereafter with the release of the original Traxxas Slash. It was durable, it was cheapish, easy to learn and hard to master.
In 2009 other manufacturers followed suit: HPI released the Blitz to compete, Losi released the Strike, Team Associated put out the SC10 based on the B4/T4 platform. In 2010 Team Durango put out the DESC410R 4WD....and so on. It was popular enough that it was being raced not just as a beginner class but as a national competition.
So it needed rules. And those rules specified a max width. It happened that was the width of most cars. But how each brand's parts add up to this width varies from manufacturer to manufacturer. Some have longer arms, but some have wheels that stick out more (offset)
Well, it so happened that the Team Associated SC10 was 6mm under the rules with the stock equipment, because it was based on the RC10T4. So Team Associated and 3rd party vendors quickly released +3mm offset wheels (3mm x 2 = 6), so that the car is more stable. The original wheels became known retroactively as +0mm Associated offset.