Why do DK's & DH's suck?

In wild why are death knights and demon hunters so worthless? I’ll hear the argument for their newer classes so in wild less cards to play with, but in theory shouldn’t all cards be equal in value depending so wouldn’t dk’s and dh’s have an equal chance? Anyway, I have a plague death knight, blood and frost with their associated best cards and once I get to silver 10 I’m lucky if I win 50% of my matches, meanwhile I have a few decks that I can pretty much count on to get to gold 10 if I want to put in the effort and play the same 3 decks 50 times.