Because focus on some (preferred) civil liberties ignoring others marginalizes the other liberties. Also, because at the same time they are fighting against other liberties and causing harm.
So you are against the NRA then, as they are marginalizing other liberties.
That's a ridiculous argument anyway though, because that means that any organization that is fighting for rights is inherently marginalizing all other rights by not fighting for them too. Taking your argument to its logical conclusion means we should have no rights organizations.
Just a cursory glance, looking at the latest ones: Wisconsin vs Mitchell is BS, Lee vs Weisman is BS, Wallace vs Jaffree is BS. And that's not even showing all the zillions of cases where they were on the wrong side attacking civil liberties (BSA), or where they threatened people/businesses/school districts with expensive litigation if they didn't give in to their demands. That's a lot of harm.
The list was up through 1996.
Lee vs. Weisman is BS? Wallace vs. Jaffree is BS? So basically you're for state endorsed school prayer. While that's certainly your right, my guess is that you're in the minority here.
I imagine more people would agree with you on Wisconsin vs. Mitchell, but when you stack that up against:
Brown v. Board - desegregating schools.
Loving v. Virginia - overturning interracial marriage bans.
Gitlow v. New York - landmark protection of free speech.
DeJonge v. Oregon - more huge free speech protection.
Hague v. CIO - kept the government from corruptly denying people the right to protest.
Gideon v. Wainwright - established the right to a competent attorney.
Griswold v. Connecticut - kept the government from banning birth control.
Miranda v. Arizona - established the Miranda warning.
Roe v. Wade - right to abortion.
US v. Nixon - should be obvious.
etc, etc, etc.
Seems like you should be thanking them, no?