There are 20 million programmers in the world, that is the maximum S.O. can get. There are billions of people looking for just the type of answers you find on Quora.
Also you go to S.O. to find a specific answer than move on, on Q you stick around and read other stuff, go back for discovery.
I am not saying which one is more valuable, perhaps S.O. is more monetizable.
I've followed many links to Quora but bounced every single time because of the stupid signup wall. Subsequently, I've banned them from my Google search results, same as Experts Exchange. SO and Reddit are sharing the Q&A lunch.
" There are billions of people looking for just the type of answers you find on Quora"
And how many of them are going there? Every indication is "not many".
"Also you go to S.O. to find a specific answer than move on, on Q you stick around and read other stuff, go back for discovery."
By "you" you mean?
I've visited quora maybe a dozen times, usually by following a link from an aggregator like HN. I didn't feel compelled to poke around once I got there, and the comments and replies (though they came from the people with higher reputation) generally ran the same gamut as everywhere else.
StackExchange has much more than just programming sites, and I've done all sorts of discovery via the "questions from other StackExchange sites" ad units and sidebars.
Also you go to S.O. to find a specific answer than move on, on Q you stick around and read other stuff, go back for discovery.
I am not saying which one is more valuable, perhaps S.O. is more monetizable.