Winter suits are rarer than those for the rest of the year. I am far from an expert on ready to wear clothing but, on those occasions that I cruise the racks at the friendly neighborhood menswear retailer, the suits are all medium weight and lighter. Heck, the last time I looked at Zegna's made to measure cloth offerings the heaviest material they had was 11 ounces (330 grams), which is what I wear in moderate temperatures. When it is colder than 55 (13 C) I reach for something warmer.
By winter suits of course I mean those with cloth in the range of 13-16 ounces (400 to 500 grams), 16 ounces being about as warm as a man can wear in a temperature space. André Churchwell's glen check in the photo is an example, with cloth that is visibly heavier than what we see in typical department store suits.
I guess we cannot blame most of the clothing retailers, and I say most because many major cities still have one or two menswear shops that stock a full range of weights (Samuelsohn of Montréal, Canada, for example, makes some respectable examples that retail for $1,000 and a bit more). Seasonable wardrobes cost more than one year-round set of clothing, and it is possible to extend that temperature range of that mid-weight stuff with outerwear. Both the retailer and the customer may save some cash, though the real cost of that approach of course is cold legs for the man wearing what is essentially a summer suit in the depths of winter.
A compromise approach that does not add much cost to the typical wardrobe is to acquire mid-range clothes and then supplement them at one end of the temperature spectrum with a couple of summer suits and a couple for winter. There are in each season a couple days that are more moderate than others when mid-range cloth is perfectly adequate, but the winter suit will be appreciated when temperatures fall below freezing. Every wardrobe should have a couple of them.