Beats me, I'd love to know. I think it's one of those things that gets repeated ad nauseam without verification. I've read conflicting sources that over modification and under modification reduces SMM (the precursor do DMS). It is linked to nitrogen levels. Although this can be affected by variety, fertilisers, grain storage and malting. So perhaps modern technology and practices have reduced SMM in malt? When and how, I don't know.
I think the more important thing to consider is brewery process. Ignoring the small amount of DMS that occurs naturally in malt. Levels of DMS in beer is largely the result of brewery process from mash to fermentation. Conversion of SMM to removal of DMS. With DMS only effectively being a non-issue if you don't go above 80°C at any point in the process.
http://www.milkthefunk.com/wiki/Dimethyl_Sulfide
There's a study that many commercial beers actually contain levels of DMS above the flavour threshold. Indicating that a degree of DMS is actually part of character of beer. With some breweries controlling how much they create, through whirlpooling, to meet customer expectation.
But, now we are heading of around for another loop of the same argument. And I'll await the usual reprimand from insecure people, for knowing something, and wanting to learn more.