Why are some acrylic yarns super soft and some not?
According to the "Acrylic fiber" page on Wikipedia: 'For a fiber to be called "acrylic" in the US, the polymer must contain at least 85% acrylonitrile monomer.' That leaves a free-for-all on the other up to 15% of fiber that goes into making up a yarn.

Although there are accepted standards for putting the weight and recommended hook and needle sizes on yarn, there isn't a standard for softness. Some companies do describe some of their yarns as being ultra soft or something similar, though.

You might be able to figure out how soft a yarn might be by reading what it's made of (if they were nice enough to list the fiber types in detail). The care instructions could also possibly tip you off. Another idea would be to visit chat rooms or crafting websites or reviews on shopping sites to get details about the yarn beyond what's on the label.

The disadvantage to relying on yarn companies or other people's opinions to tell you how soft a yarn is, though, is that softness is subjective. I may tell you that yarn X is incredibly soft, but you might not think so when you get your hands on it.

Probably your best bet would be to keep the name and other details of yarn(s) that you like the softness of and ask for comparisons between that and the yarn(s) you're thinking of getting. That way, if a person says it feels and/or behaves very much the same as your yarn, then you can be pretty sure you're getting something you'll like.