Because Quora, at a corporate and (mostly) community level, regards itself as an authoritative source of information, rather than a social forum: it deprecates the “interactivity” you speak of, as detracting from the authoritative tone it aspires to.
You will have seen a lot of answers that agree with this judgement. And a little more critically than that, the advertisers likely to want their ads promoted here will also agree with this judgement. (Scott Welch’s answer to When do you think Quora is going to end?)
This doesn’t mean Quora, or the harrumphing answers here, are right. It’s all conventions, they all change, they’re all socially contingent. But right now, in 2017, emojis have an association of levity and unseriousness, and non-emojis have an association that Quora and its core audience seek, of seriousness and expertise.
They’ll wrest my emoticons from my dead clammy hands though. 😐
I’ve spoken elsewhere of the Silicon Valley notion of what scholarship is all about: actual scholars can crack jokes. Just look at the terminology used in computer science. But for now, that’s the register, that’s the prejudice, that’s the expectation.
(Oh, and of course you can actually enter emoji in the editor: [math]unicode{x1F61B}unicode{x1F69E}.[/math] Do expect Quora Moderation to come after you if you do, though.)