What's the difference between humans and orcs?
When you get right down to it, beyond a change in skin color, how are Orcs significantly different then humans? Why do Dragonborn get your dander up when 'Humans with different skin color and maybe ear/height changed' dont make you bat an eye?
Who said all the humans in my campaign treat each other or demihumans differently? The people in the "home country" of my campaign look down on the rest of the humans, and are in a constant state of war with both their northern neighbours. They'd also likely try to kill any armed party above a given size of their southern neighbours, steppe-dwelling nomad clans, if they crossed the border, just on account of past raids. Elves & half-elves are distrusted in that country, in other countries, people attempt to enslave them on sight. Not that there are many elves around anyway. Dwarves would be battled if they were not living mainly underground "out of the way" of the human nations. Halflings are living on the fringe, in their own lands, avoiding most of the humans. Gnomes... haven't had a gnome NPC for years, so they can be on their own island for all I care.
There's one country where all "civilised races" (HUmans, dwarves, elves, halfelves, halfings) are equal (humans dominate though), but only as long as everyone worships the four elements according to the prophet's law. Most "infidels" and all "uncivilised races" (all the other humanoid races) are to be enslaved for their own good since they're no good for society otherwise. Orcs though are killed on sight, not enslaved - the relam remembers the horrors of a past horde.
Most of the enemies of my PCs are actually humans.
So, lizards are not singled out, just treated even worse for being so different - that is, they'd be treated differently, if they ventured out of their swamps. Of course, having a snake god be the leading evil god and main enemy of the campaign's central country's pantheon and having Yuan-Ti meddling against the realm doesn't eactly help with acceptance of scaly humanoids as civilised races.