Pop radio has been in different phases in the last decade...
1999-2001 is when you had the peak of the pure pop era with acts like Britney Spears, Christina Aguilera, Backstreet Boys and N*Sync
2003-04 is when hard rock bands like Evanescence, Trapt, Lostprophets, and Crossfade broke into top 40 radio.
Between 2005 and 2007 is when you had a lot of dirty south style rap artists breaking top 40 radio like D4L, MIMS, Yung Joc, and Lil' Jon.
2008 was the year of Pop&B with Chris Brown, Rihanna, and Usher dominating pop radio.
Ever since Lady Gaga's prominence in 2009, radio has sounded a lot like this.
Why? Putting a sociopolitical spin on it, the recession hit the US in late 2008 really hard and we're just now starting to get back to some semblance of normalcy.
Dance music seems to be the perfect escape music, so it's not entirely surprising to have seen acts like Taio Cruz and LMFAO dominate the Top 40 scene. Even songs that weren't meant to be dance songs in the first place are getting remade for the floor and are getting a lot of airplay, such as Adele's Someone Like You.
It's gotten to the point where other genres of music have had to adapt to the dance craze. Jay-Z & Kanye West's Watch the Throne project released the dubstep flavored "N**gas in Paris" as a single and Korn put out an entire album influenced by dubstep, The Path of Totality, last year. Add the three Grammies that dubGod Skrillex won a couple of weeks ago and it seems like this dance influenced pop music is here to stay for a little while.