Why was the American Federation of Labor (AFL) called the "bread and butter" union?
Likely because it concentrated on wages and work conditions rather than anti-capitalist politics.
It was a conservative alternative to the more radical left Unions that challenged the capitalist system. They were dominated by the Trades and Craft Unions. They were interested organizing across industry skilled trades rather than the mass of unskilled labor. This pragmatic view limited membership and thus political influence.