As someone who grew up in the '60s/'70s, the biggest change has been - as touched on in the article - that you don't need (or need as many) office-specific and "event" clothes. And it's not just not needing "business / office" clothes - although, that is a part of it.
Growing up and into the '80s (and even the '90s), you needed nice "dress up" clothes for social events like weddings, dinners out at nice restaurants, funerals, etc. While not as nice as in its heyday, people did dress nicer on airplanes or even going over to someone's house for dinner. While some of that still exists - it is a meaningfully smaller universe today.
And even though your business clothes could "double" for these events, you usually kept a few "nice" things aside for those "special" events. Also, you might want your go-to wedding-guest suit to be a bit less "businessy" than your office suits. Maybe, you'd also get a more "interesting" tie or two or a pair of "different" dress shoes.
And there were all levels and grades along the way. You'd need a tux (and all that entails if your life had those type of events, which was much more common for regular people - like me), the aforementioned suits, etc., for work and weddings, but also, sport coats, dress slacks, a range of sweaters, etc., that were a touch "down" from suits, but well above jeans and other truly casual attire.
While many people still have a suit and a few ties today, the number of items needed are down dramatically. The same with shoes where I have friends that "keep" one pair of dress shoes when needed versus when men truly needed several pairs of dress shoes.
So, today, a few items cover the few times you need to really "dress up" and everything else is casual / business casual. Maintaining a casual work wardrobe of khakis and/or jeans, polos and/or Ts, sneakers, etc., (that also do double duty away from work) requires fewer total items and, overall, less-expensive items.
While (I'm guessing) I still have more than the average number of "dress up" clothes, my wardrobe is down from the '80s as there's, for example, no need to have a range of summer sport coats that never get worn as, even in NYC (which is still a bit dressy), in the summer, almost no-one wears a sport coat. And most business meetings I go to don't call for a suit, so my total number of suits (and all that goes along with that) is down.
To be sure, I still have a lot of clothes 'cause I like them, but my needs are meaningfully down from the '80s as, I expect, are most people's. Hence, as noted in the article, if the average person now spends ~3% vs ~6% (in the '70s) of their budget on clothes, I'd say that almost feels high as the suit / "dress up" / fine clothes part of one's wardrobe was the most expensive. I can buy a lot - a whole lot - of chinos, khakis and OCBD and still not spend what I would to have a working rotation of suits, dress shirts and ties for regular weekday wear. So, maybe I need more causal clothes than before, but that just means I'm replacing expensive clothes with much-less-expensive clothes.
Yes, there is a valid discussion around income distribution, economic trends, etc., (although, the economy was in the trash can for most of the '70s), but, all other things equal, our culture simply requires not only less variety in clothes (hence, fewer total clothes), but fewer of what were the most expensive ones.