You can quickly find historical availability & consumption data and I don't think it supports any trivially obvious hypotheses like these. You'll find headlines saying things like that we're at a low point in vegetable consumption going back to 1988, but I'm reading an NIH paper charting '70-'2010 and the patterns look stable, except for increases in total calories, in dairy, and in added dairy fats and oils.

Whatever's going on, it's probably going to end up being complicated and multifactorial.

(I do love me a crucifer, though).