Why do plastic tank graphics fail

The manufacturing methods and materials are different between the two stickers.

Put that VP surface printed sticker on your race bike and your knees will rub the logo off in two days of riding.

MX graphics are printed on a sticker sheet with extra thick and gummy adhesive, then coated with a very thick clear top-sheet for wear resistance.
The cheap gas-can stickers have a thin adhesive and they don't have that wear-resistant clear top-sheet.

I'd imagine a company like 3M could solve that problem, but likely it's not worth their time to develop that niche product.

What material is it your gas can? The problem with gas tanks in PE plastics is that gas fumes leaks through the material and need to be ventilated through the graphics. That's the purpose of the perforations. 

If they're made of different plastic, that could be a factor for sure.

Motocross shrouds, fenders, number plates, etc are often marked PP (PolyPropylene) and I always assumed the fuel tanks (on the bikes) are made from the same material.  It's entirely possible that they are PE (PolyEthylene) or HDPE (High Density PolyEthylene).

I just looked at the bottom of one of my VP fuel jugs, it is marked HDPE.

 

The volatile parts of the gasoline will permeate the plastic gas tank and jerry can. Those volatiles probably get trapped under the thick bike graphics but permeate right through the thin vinyl of the sticker on the jerry can as if it was a part of the underlying plastic. The bike graphics are laminated layers of different materials while the jerry can stick is a single layer of vinyl with a graphic printed on it.