[Mauricio_]

Probably because most SO questions can't be answered with product recommendations.

Also SO is very specific and narrow (programming questions only), while Quora is very broad, so it's probably harder to moderate effectively.

[OJFord]

And before someone says 'Stack Exhange is just as broad as Quora' -- all the SE sites I've come across disallow 'product recommendation' questions or whatever similar thing fits their niche.