The main reasons I can think of for the 2 grid wide standard are simply convenience and practicality.
It is often easier to fit a 2 grid wide cage into living spaces in a manageable and practical fashion. Also for shorter individuals, or for individuals who have a somewhat limited range of mobility, (particularly if the cage is raised off the floor) it can be difficult or impossible to reach all areas in a 3 grid wide cage to properly clean it.
If you are able to accommodate a 3 grid wide cage and maintaining it is manageable for you then there is no reason not to chose a 3 grid wide instead of a 2 grid wide, in most cases guinea pigs do greatly appreciate the extra width.