Because the traditional family is in the way of the State as Daddy. Mao is a great example.
You don't get more left than the infamous Martha Burke.
She is the feminist who wanted to force a private golf club to accept women and failed to get it kicked off CBS.
She advocates abolishing marriage and having reproduction controlled by female only fertility panels. It would regulate male fertility and thus abrogate marraige.
Her works have been posted here on FR in the past.
She is just the tip of the iceberg in feminist studies programs at all universities.
Hitler had birthing villages as part of the states effort to produce perfect aryans.
the USSR was militant in making children obedient to the state. Soviet propaganda made a child who turned in his farmer parents to authorities for withholding some of their crop. The village killed him after the parents were arrested.
The writings of all leftists/communists/socialists are repleate with the fact that they need to eliminate the family unit in order to make loyalty to the state complete.
Every child born in a mother and father family is a defeat for the left.