In a show like this where the characters are generally more known to audiences and fans of the lore, it's sometimes better to avoid having a recognizeable face in a big role.

Tbh, I would venture to say in HOTD 99% of the people watching haven't seen D'Arcy or Cooke in anything. And even someone like Ifans is a bit more of a character actor and I'm willing to bet most people didn't realize he was the Lizard in the Amazing Spider-Man.