The reason is simple... some among us, like me, don't like to see monsters become player races. And yes, for me even Drow and Duergar are already pushing it.
Why can't people just play an ordinary elf, halfling, gnome, dwarf or human? Why does it have to be something outlandish like Tieflings or Dragonborn or even worse, a monster race?
Yuan-Ti are monsters for a reason, same as goblins, kobolds and every other monster race someone may ask to play.
You say it's for a reason... but then you don't actually give that reason. What is the 'reason' you're referring to? Care to explain?
Why should monstrous races be an option available to Dms and potentially players? Because, for mortal races, evil is cultural not in-born; individual exceptions, through fluke of circumstance and opportunity, can and do exist, and it's important that they are allowed to. The part where most attention to this falls down is that it takes these exceptions and makes them bland and generic, because everyone and their dog just generally and immediately treats these rare and exceptional individuals as 'just the same' as all the other adventurers, right off the bat. There should be hesitation ,trepidation nad mistrust from at least a large portion of people; they are likely to have trouble being allowed in some towns (not everywhere, and a good DM ensures that it's never an absolute lock out or debilitation that party, as a whole, cannot overcome, but is should be a present issue that acts as a force upon the character, and lets them express themselves in how they deal with it).
In the case of yuan-ti, 'purebloods' is a derogative, insulting term, and purebloods are the lowest of the low, in yuan-ti society, because they are basically human (or whatever other 'normal' humanoid race they owe part of their lineage to), with virtually no snake-like elements at all; they are very close to being an offence to all that it is to be yuan-ti, at least in the eyes of their own society. They can be successful if they're useful - as spies and infiltrators, often, but otherwise, they're the lowest and worst-treated caste of all and are taught to loath and hate themselves for the 'purity' of their blood, and to seek other ways to snake up a bit more... So, occasionally, you do indeed find cases of purebloods to look on other civilisations, and think it seems like a better choice.
They *tend* to still carry a lot of the values and lessons of their society with them, even so - they very often still have incorporated a lot of the ruthless, selfish and emotion-minimising traits and values that yuan-ti society teaches into their own world view... but not always, and nevertheless if so, these individuals, rare as they are meant to be, can still find places amongst adventuring folks.
But it is not easy for them - and that's an important part of what it's meant to be, to play a monstrous race character trying to be different.
The important point to be made is that this doesn't risk the 'we still need evil things to fight' problem, as long as the player characters that do this are noted to be unusual exceptions to their otherwise still very much evilly-inclined cultures and societies; those are still there, or they should be... Unfortunately that crucial element is also the bit that modern day Wizards seems determined to erase... so... *shrugs*
