Instead of paying royalties for 30 songs, they'd rather not pay any royalties at all.
Royalties are not based on how many songs you play. Depending on which major licensing service you're talking about, a station's costs are computed either on their cume or on their billing. It costs the same to play 3,000 songs once or 1 song 3,000 times. So stations do not shrink their playlists to pay lower royalities.
The only way they can change what they pay is to stop playing music.