I know the idiom motherhood and apple pie is used to denote some principles with which few disagree. But what is the connection between motherhood and apple pie? I am not very familiar with American culture. I want to understand what makes this phrase to mean what it means.
I don't think there is any connection, other than their being two things which no one could possibly object to.
(Not really relevant, but apple pie is also used in the expression 'as American as apple pie'.