Part of the reason for quora to do that might be to limit their user base to people who already know what it is (people who are referred)

Exposing what it is and making it really easy for anyone to sign up will make it degrade into yahoo answers in no time.

Stack overflow on the other hand embraces new programmers asking and finding answers to questions because they're all going to be related to programming (if not it gets modded down really quickly). Quora doesn't have that niche/topic restriction and is much easier to troll.