The motivation is to win and the political philosophy of those starting their political careers will differ from the establishment.But clearly the ruling conservative parties don't actually have any reason or motivation to advance laws that benefit working people, and have demonstrably done the opposite. Very puzzling how you could imagine conservative parties (which are just as entrenched in finance capital and large industry as the Democrats) somehow turning into working class parties.
AOC as a figure may have varied popularity, but her policies don't. Medicare for All is extremely popular, even among many Republicans, and so is the Green New Deal among voters, but these policies have no champion in control of either party. And of course we have an electoral system that protects against majoritarian outcomes.
This is something that gives a good example of what I am talking about.