Here's a list of 2012 Billboard Hot 100 singles, for reference:
'Sexy and I Know It' - LMFAO
'We Found Love' - Calvin Harris ft. Rihanna
'Set Fire to the Rain' - Adele
'Stronger (What Doesn't Kill You)' - Kelly Clarkson
'Part of Me' - Katy Perry
'We are Young' - Fun ft. Janelle Monae
'Somebody That I Used to Know' - Gotye ft. Kimbra
All of these have an inherent listenability to them. And yes, listenability isn't really a word, but it should be, dammit. They're all easy to listen to, follow a set musical formula with little-to-no deviation. The first four have already established fanbases, but the last two are interesting in having driven up interest through other means - Glee and commercials for Fun, and YouTube for Gotye. Like the movie industry, it appears that 'hit' music is becoming more and more established on known-properties that either have a lot of success already behind them directly or indirectly. And then of course there's the radio, and the fact that people who buy singles are more likely to base their decisions on what they keep hearing on the radio, as opposed to reading album reviews or generally stepping out of their comfort zones. 'Cause if more album-buyers bought singles, the charts would look very different indeed.