The ACLU did tackle Citizens United: we argued in favor of that non-profit's right to free speech. That the Supreme Court extended that ruling to lift restrictions on corporate contributions to uncoordinated campaigns was, of course, far outside the ACLU's limited 1st amendment agenda. The organization is still discussing its policy on contributions as speech in light of the new ruling.
The ACLU is currently focused (and fully booked up - thank you JayB for actually recognizing real world constraints for once) on drug policy reform, privacy, equal protection for committed couples, and several other issues more core to a human rights focus than property rights.