One important one is impact. Most people treat Wikipedia content as "probably true", so any time where the truth of something matters, zero content is better than bad content. That's true for all sorts of scientific, medical, and commercial content.
Another is impact on living people. The folks who wrangle the rules for biographies of people still alive [1] have put a lot of thought into recognizing the impact that Wikipedia articles can have on people. Having nothing is often better than having a bad article.
A third is self-promotion and vanity. People are willing to write endlessly about themselves. Bands formed by teens in their garages, companies trying to promote themselves, blatant spam, subtle commercial manipulation: Wikipedia doesn't need that.
A fourth is maintainability. The number of active Wikipedia editors is modest. Some sorts of content attract new editors. But quite a lot won't. I think it's often better for Wikipedia to have no article than a bad article that isn't going to get better soon.
And that points toward a fifth thing: the value of the Wikipedia brand. Wikipedia won the reference race because of its quality. It is enormously valuable for the world to have a single, high-quality factual resource. The brand should be maintained. People can (and certainly do) disagree on where to draw the line between crap and not-crap. But almost nobody disagrees that there is quite a lot of non-encyclopedic material (e.g., [2], [3]), and that the Internet is plenty big enough to host that somewhere other than Wikipedia.
[1] http://en.wikipedia.org/wiki/Wikipedia:Biographies_of_living...
[2] http://en.wikipedia.org/wiki/Wikipedia:Criteria_for_speedy_d...
[3] http://en.wikipedia.org/wiki/Wikipedia:What_Wikipedia_is_not
>Wikipedia won the reference race because of its quality.
Weren't convenience and quantity important factors and more important ones? To me the value of Wikipedia is that virtually any topic is discussed reasonably consistently with reasonable quality. If I specifically need quality, I'll look to the article's sources or find my own.
Another instructive place to start reading about WP quality is the Featured Article process; in particular, try clicking into some of the discussions about individual FA candidates.
Compared with whatever Google coughs up, I think its big win is quality. Other Google links are equally convenient, but Wikipedia is trustworthy. And the big Wikipedia controversies have mainly been about quality failures, so I think that's most significant to readers in terms of maintaining brand value.
I agree that the necessary level of quality here is only "reasonable quality" rather than perfection. And I agree that other factors definitely helped.
Here's yesterday's:
http://en.wikipedia.org/wiki/Wikipedia:Articles_for_deletion...
What logs like this tell me is that if "deletionism" is really a force on Wikipedia, it has a clear cause. Note the sheer volume of marginal (or sub-marginal) articles WP is dealing with, and also the degree of attention each one of them gets. Click through to the article on "Samarqand Restaurant", and then back to the AfD debate, and note that WP people actually find reasons to stick up for it.
I've participated in a large number of AfD discussions, and the vast majority of the articles that get deleted simply didn't belong there in the first place.