My guess is that they didn't think of it and no one has requested this feature. Here's a link to a list of feature suggestions submitted by active Quora users:

http://www.quora.com/Quora-User-Feedback/What-features-would...

Of all the suggestions, Khalid's suggestion seems most relevant to yours:

"I just signed up, and was a bit horrified to see 40 odd possible e-mail notifications I could receive. But I was disappointed not to see a link to an RSS feed of those notifications." (http://www.quora.com/Quora-User-Feedback/What-features-would...)

Note that his main suggestion is to have Quora provide RSS feeds in place of email notification as opposed to adding a button that lets users disable all email notifications.

Like I said earlier, their sensible default settings make this feature somewhat unnecessary.