It's more about the psychology of the "uncanny valley" than anything. When something looks "almost human", it's creepy. The Argonians in Oblivion kinda freaked me out with their oddly human features like abs, pecs, briasts, no claws, etc. If they made them look more like lizards, I'd find it easier to be less repelled by them for being so... human. People are supposed to look like people, once some race with lizard eyes, scales, and a long tail starts to intrude on that mark it makes you judge their appearance more on a "human" level than a "non-human" level. I can empathize better with the Shrimp-Like people from District 9 than I can with a human with scales, red eyes, spines growing out of their heads, and long tails 'cause I see them as a person with odd features, not a race different than humans.