Consider the regions of the country where states have been more likely to legalize marijuana (in various forms) compared to those that have kept it illegal. in what way does this illustrate a strength of a federal system?

I think it illustrate the strength of the federal system. States that already legalized marijuana are considered the largest states in united states and contributed to the majority part of its economy (such as California).
If the feds really want to show the strength of its power, it need to make sure that all states follow one guideline that they imposed