Alternative framing: Given limited resources and lots of things to care about, they pick the specific cases that best improve the freedoms they're interested in protecting.
In the case of the Second Amendment, they decided to let the NRA handle it, as that seems to be working just fine.
The ACLU should defend people who suck ass and another group should defend the heroes who beat their ass for saying awful shit.
So maybe you pick the anti-ICE protester instead of the Nazi to help out. Both got shot with pepper balls, both had their rights infringed upon. Why not pick the one who isn't a complete ass to establish the same precedent with?
They've explicitly said the opposite.
https://www.aclu.org/news/civil-liberties/defending-speech-w...
2023: "We joined Young Americans for Freedom, the Cato Institute, and other unlikely partners in filing an amicus brief on behalf of the Foundation for Individual Rights and Expression in its challenge to New York’s new law regulating 'hateful conduct' in social media."
The Second is probably the amendment least in need of defending by the ACLU. It's well covered, and pretty much a third rail of American politics.