Conservative Christian fundamentalist groups have, ever since Reagan, likened a national ID card to "the mark of the beast", citing Revelation 13's prophecy that no one could buy or sell goods without the number of the beast, and the reference to God's chosen people that refuse the mark of the Beast.

When a small but meaningful fraction of your constituents fervently believe that giving them a national ID literally damns them to Hell, it's a hard sell.

Much easier if you attach it to a tool (Social Security) with which they can get money.

There's also the angle of State's Rights and whether the federal government can require people to have national ID over just amalgamating the various state IDs.

Finally, Americans generally distrust their government. Not without reason, they believe a national ID would reduce their privacy and allow the government to more easily spy on and track them.