How can you ask a parent which among the children is the favorite? This one's the cutest, this one's the smartest, this one's the most sensitive, this one's the most successful, this one's the most dignified, this one's the most mature, this one's... you get the point. Even if selecting a favorite is possible (certainly it's not diplomatic) it's liable to change from day to day and mood to mood.

Therefore many abstain from voting.