Scenario: You're listening to Adele sing "Rolling in the Deep" at the Grammys, and you're noticing that she's really good--transcendently good, one might say. One might then hie thyself to Spotify--let's assume here that you're in the much narrower demographic who knows Spotify but hasn't yet considered searching for Adele--but one would then notice that 21 is nowhere to be found. (Then, one probably would have bought the album instead, because there are apparently hundreds of thousands of people who tuned in for the Grammys and are willing to buy albums yet somehow missed out on Adele's nationwide blanketing of public and private space with "Rolling in the Deep" yet "Someone Like You." If you are one of these people, please send us an email, because it blows our mind that you exist.)
There's no way this wasn't on purpose, right? There's no way Adele's team simply forgot that there'd be a massive surge of interest in 21 after what was always going to be Grammy triumph, or forgot that streaming music is always right there, one icon away. And yes: Fast Company reports that there was no way.
The full details are at that link, including more info on how it might affect 21, but the gist: Adele didn't want people to be able to hear 21 for free (Spotify's entire catalog is free but has ads, which people can pay to get rid of), Spotify didn't want to split its catalog into free and paid music, and therefore, Spotify didn't add 21 to its offerings, although other services like Rhapsody did.
So now you know. And as 21 continues its seemingly endless reign, you'll know why Adele made the call she did. It's more tempting to buy an album with one fewer alternative.