My daughter has celiac disease. Finding gluten-free kosher food isn’t alway easy. And finding gluten-free foods that she likes can be a real challenge, because many gluten-free foods don’t taste as good as their gluten-containing counterparts. I’m not going to limit her options further by restricting her to a c”y diet.
I used to buy c”y milk, but stopped when I found it spoiled within a couple of days of purchase, long before the stated expiration date. I rarely have that problem with cholov stam milk.
Another reason that people may not use c”y exclusively is that c”y products tend to cost much more than their cholov stam counterparts, and are rarely discounted.
Of course all of this is assuming that one’s rav (as ours does) permits the use of chalav stam.