I found this question on Quora.
Here’s my speculative response:
I’ll hazard a guess.
The Quora team needs metrics that help them find good questions to put in front of the audience.
Now while you’re looking around, you might think a question is good. But the Quora team might define good differently than you.
So they only give you the tools that help them. If you had an Upvote button, you might Upvote questions that weren’t that great after all.
Instead you have the Follow button. Follow is for when you think a question is good enough that you’d ask it too. If you think a question is that good, then it must be pretty good.
Likewise, if you think a question is good enough to answer, it’s probably good, too.
So they have Follows and Answers as two metrics. If they added Upvotes, that would be the worst metric of the three.
There’s more. Upvotes would discourage you from thinking about using one of the other two tools. Right now, if you like a question you have to think to yourself “Do I like this question enough that I’d want to read the Answers that get added?” If the answer is yes, then you hit Follow. If the answer is no, then the question isn’t worth your time. And it might not be worth anybody else’s time either.
If you had an Upvote button, you wouldn’t have asked yourself this question.
Finally, a Question kind of does have Upvote buttons in the Answers. Since the Question owns its Answers, all the upvotes on the Answers also count toward the question. Upvotes on Answers are probably a good metric.