Slap some boobs on 'em so they'll sex up the art. The boobs are only there to make the books more attractive to 13-year-olds.
I'm like 358% certain this wasn't a deciding factor, honest. I'm also pretty darn sure that no one went with pawsplay's ninja boobie turtle pic above because they thought it was "hawt".
It's just a style of anthromorphism that gets used a lot. Just looking at 3rd edition, I've seen Yuan-ti, Medusa, Half-Dragons, and
Dragonborn* with boobs, and I sure I'm missing some examples.
Yes you can argue that all of the above creatures are part human, ect ect and that makes it ok, but the simple fact of the matter is, that isn't why they get drawn with boobs and Lizardfolk don't. The manager of art direction at wizards does not have an official boob and scales policy that stipulates the comparitive ratios of lizard and mammal blood were boobage is officially acceptable.
It's because Lizardfolk and Troglodytes are less anthromorphized then Half-Dragons and sundry. Lizardfolk bodies are more lizard shaped, whereas Half-Dragons and Dragonborn and Medusa and all their cousins have human shaped bodies. This is the deciding factor. Now, I'm not saying you have to like the design for Dragonborn or there is anything wrong with wishing they looked more reptile-ish. I just get tired of people ascribing silly motives to the designers without any evidence.
Heck, this kind of thing is done all the time, even to characters who aren't humanoid to start with. I don't even know how many 4 legged disney animals I've seen that have conveniently shaped puffs of fur to make them like more feminine to the audience. The existence of furries on the internet does not make all forms of anthromorphism sexually charged.
I can't believe I let myself get sucked back into this thread...
*Original version of Dragonborn, where they were basically transformed humans and demihumans.