I think its because Diffindo wasn't shown as a horrible and dark spell. As for Sectumsempra there was a whole scene where Draco was attacked with it.

We love everything horrible and Dark! /j

^facts /jk

Draco did not need the trauma of being hit with the spell. He already had enough to deal with.

I AGREE WITH YOU^

We all know harry's reckless, but he tries to be a good guy

Def not a smort boi tho

'hmm, a spell that says 'for enemies'! i don't know what it does, but i can certainly test it out!' lol

^literally what Harry was thinking

Lol