Alexa is popular because it's been there for a long time and is well referenced. Been there since 1996. Not accurate because it counts only those websites that have installed its toolbar. It’s non-inclusive. Currently (end of 2013), there are nearly 500 million websites, but only a certain segment, like webmasters & techies, has installed the toolbar. Reduced credibility as they get data only from their toolbar. Biased, as only websites with Alexa Toolbar will rank at the top. It doesn't come pre-installed on any major browser.
When you finish reading this presentation you will be able give expert opinion on how Alexa ranking clouds decision making.