I wonder if it is the books I read, but I find these two words quite different and both useful. Yet the Grammar Police would have me believe one is no longer used (link).
I disagree. After the explanation I will use them both in similar sentences and you can see the difference.
The Count glanced at the thief with disdain before moving on.
The Count looked at the thief with distain, shuddered at the visage, and moved on.
Distain, you see, is a more active loathing, where disdain is more passively thinking "it" is beneath you.
This came up for me in writing a review for The Hairy Ape. He is looked at distainfully - yet this was flagged as incorrect and my electr4onic editor wants me to use disdainfully.