I think the site owner actively filters them out and has been doing so ever since you joined this site in 2011.
But no matter the reason for the complete lack of emoji, the fact that you like this site enough to participate here is evidence that emoji don't actually improve public discussion as much as you think they would.
There are many many sites on which you could have chosen to participate where emoji are common and can be used freely.
You choose to participate here instead of having face-to-face conversations. Does that mean that face-to-face discussions aren't useful? I walk on sidewalks where cars are prohibited. Does that mean that cars aren't useful? I drive on highways where pedestrians are prohibited. Does that mean walking isn't useful?
Whatever point you're trying to make, this is an extremely ineffective argument for it. You’re in “checkmate, atheists” territory here.
Isn't there a comic about this kind of argument? I don't think emojis add anything, but lack of emojis on HN doesn't say anything about that, other than the creator probably doesn't like them either.
I also don’t use contractions in formal writing settings. Does that make them bad?