Why do あなた think some songs on glee/グリー have been released as singles または in the albums and not others?

For example: despite Rachel 歌う 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 あなた 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 秒 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 あなた think that is? Tell me what あなた think!