In fact, they've suspected this for a little while and have been posting videos on their YouTube page showing those poor helpless millennials how to use fabric softener (as in the video above). How kind of them...
I'm guilty of cracking many jokes about millennials, though many of my friends strongly identify as such. (I'm on the cusp, and really think of myself more as an X-er, but whatever.) Still, the whole "oh you don't do this? maybe you don't know how" bit has been true in many different contexts. Everything from pushing for better hand hygiene to giving a better job interview has been responded to with education. And it's possible that lack of knowledge is part of the reason millennials are turning their nose up at fabric softener, just like a lack of knowledge may explain why people don't wash their hands as well or as often as they should. But dissemination theory provides many reasons innovations aren't taken up. In fact, one of my favorite theories about diffusion of innovations comes from the work of Everett Rogers, who offers a set of variables that determine whether an innovation is taken up into practice:
So millennials may not use fabric softener because they don't see the benefit (which could be responded to with education), but the issue may also lie in the softener itself - they may not like using additional chemicals with their clothes, and therefore see it as incompatible with their values. Or maybe they have used it and didn't notice a difference, making results unobservable.
For me personally - I have really sensitive skin and I find fabric softener very irritating. (I also have to be careful what kind of detergent I use, as well as what fabrics I wear.) And the smell of fabric softener is way too strong for me. I can smell it on my clothes all day and it makes me gag. Considering the increase in allergies, which has been attributed to increased use of antibiotics - allergic reactions are an immune response, and paradigmatic changes in immune function could definitely explain why we're seeing more food and environmental allergies - it could be that many millennials have more sensitivities as well.