It's marketing. In one of Wizard's lore videos, Perkins admits that the tiefling homogenisation is due to marketing because something that looks like anything can't be trademarked. (But really, Wizards of the Coasts have never really gotten tieflings and has always been "they need to be more devil stereotypes" even as far back as 3e.)Tieflings are a good example of this, in that 5e tieflings are (in the phb and mtof) very "major key" devilish, whereas Diterlizzi's tieflings were more "minor key," if that makes any sense.
And I will say I'm of the opposite opinion about Joy Ang's DM screen. I'd gladly have that on my wall and she should be hired to do character and creature design for a DnD animated tv show.