Has it ever been otherwise?
Their initial purpose was to enforce/gather the excise taxes on liquor and tobacco, and later included the transfer tax on class III weapons as well.
IIRC, Lloyd Bentsen was the one who essentially declared the street sweeper and other firearms to be "destructive devices" under the ATF regs during the Clinton Administration, and he was Secretary of the Treasury then.
I agree that the NFA of '34, GCA of 68 and a host of other infringements ought to be repealed, but the best opportunity we have had for such in decades was wasted by the former majority holders in both houses. Pity.