Why do you think some songs on Glee have been released as singles or in the albums and not others?

For example: despite Rachel singing both 'Taking Chances' and 'Crush' for only a matter of seconds, both songs were released full length (both as singles and on the albums). And why haven't any of the Acafellas songs been released as singles (Poison; I Wanna Sex You Up)? Mercedes sang 'Don't Make Me Over' as a background song (it can be heard when Kurt is giving Rachel a makeover in Hairography) and it was released on the second album, but Puck's background song (La Camisa Negra, it can be heard in Acafellas when Puck is talking about cougars) has not been. And some songs (such as Tina's solo 'Tonight') have not been released at all. Why do you think that is? Tell me what you think!