Streaming music raises a lot of complicated issues. While some bands look at it as good exposure and an alternative to illegal downloading, others see it as another sales-reducing trend that ends up hurting the bottom line. It's not a clear-cut issue, and both sides have solid arguments. It's understandable that some artists—especially the ones with massive sales numbers—wouldn't want to have their albums streaming online for free.
Adele is the top-selling artist in the country right now and 21 was the best-selling album of 2011, so the fact that it isn't on Spotify makes sense, but what's interesting about this is that Adele and her label did not ask to have the album removed. In fact, they wanted 21 to be available in its entirety on Spotify, but under one condition: the album would only be available to premium customers (those who pay a monthly fee for the premium, ad-free Spotify service).
Multiple sources have confirmed with Fast Company that Spotify denied the request and opted not to feature the album at all. Their successful business model is one based off of converting users to the premium accounts for ad-free service, but they did not want to split the content between the free and paid service, not even for Adele. The album is, however, included on Rhapsody, where all users pay.