I'll take a stab at it... These are a few random semi-thought-out guesses.
- Consider the source; by that I mean an awesome 4Runner board with a focus on off-road. IMHO there may be some consideration given to "if the front fits, so should the back". In rocky trail scenario it seems to me a wider rear track might translate to more gashed sidewalls etc.
- If the rear was substantially wider (downright extreme)I can see how in a sliding situation it may be more likely to flip than if the tracks were reversed (wide in front). I could also argue that a slight difference like you are considering may help stabilize the vehicle. It's a toss up in my mind when you're talking about 1.25"-1.5" spacers. There's plenty of Beemers, Porsche's etc that are wider in than back.
- The final thing I can think of is rain. Depending on many factors the spray off the fronts may promote hydroplaning.
You asked "why not?" I have thought about and would gladly put just Spidertrax rears on mine. I am not super motivated but if they fell into my lap I'd try it in a heartbeat.
:cheers: