Why do they play different versions of songs on the radio?

To be more clear, I am talking about songs that are mainly singing but feature rappers. Like E.T. has Kanye West, but whenever I hear E.T. on the radio they never play the Kanye part. Same goes for B.o.B. on Price Tag and Snoop Dogg on California Girls. This might also be true for Ludacris in Taio Cruz's song. What is the point of cutting out these parts?