Politicians love to have easy targets as it deflects from their own failings. As the largest US company, Apple is the juiciest of low-hanging fruit already for their financial and tax arrangements, the last thing they need is to become antitrust public enemy No 1 as well.
Nobody outside of tech cares very much about these issue. You can build your re-election campaign on "I stopped the big bad chemcorp from poisoning our children/I stopped the big bad environmentalists from killing jobs". You can't build a re-election campaign on beating up a target your constituents don't care either way about.
Nobody except Apple’s legions of fans and loyal customers. Only the predatory companies who salivate about exploiting those users are so eager for any chance to cut down the tall poppy.