Very odd. Googling around I found a lot of coverage of this issue but nothing to explain why politicians aren't supporting it.
My best guess is that the nitty-gritty of the proposed legislation would prevent the US from funding 'bad' people who are busy fighting 'worse' people. Now, I grant you, the US doesn't have an awfully good history at identifying who falls into what category, but I understand that politicians wouldn't want the door closed entirely.
Washington's Blog 28 June 2017 Region: USA Theme: Law and Justice, Terrorism, US NATO War Agenda
The director of the National Security Agency under Ronald Reagan Lt. General William Odom said in 2008:
"By any measure the US has long used terrorism. In '78-79 the Senate was trying to pass a law against international terrorism in every version they produced, the lawyers said the US would be in violation."
General Odom is absolutely right
And because the U.S. itself uses terrorism, it's very hesitant to get others in trouble for using terrorism.
And that's why Congress is refusing to pass a bill agreeing to stop funding terrorists. Specifically, Senator Rand Paul and Congresswoman Tulsi Gabbard introduced bills to whose simple name accurately describes what they want: The Stop Arming Terrorists Act.
But neither bill looks like it has any chance of being passed right now:
The House bill has only 14 co-sponsors, and was given a mere 6% chance of passing by Skopos Labs
The Senate bill has NO co-sponsors, and was given a measly 3% chance of passing by
Love you, too
