Do you know why there weren’t any Slytherins joining the D.A.? Because they were already studying and doing DADA spells in the privacy of their own dorms and common room. Do you really think for one second that a Slytherin was going to take Umbridge seriously with her theories-only teaching method? Not on your life. They were going to smile and agree with the ugly toad, and then practice behind her back WITHOUT having to risk breaking curfew and bringing down Umbridge’s wrath on them. Easier time at school and still learning.
Potter could have learned a thing or two from them.