Not just politically difficult, it's also generally unwise. In the event of deteriorating relationships, you'll suddenly lose access to what you've been funding

Not just politically difficult, it's also generally unwise. In the event of deteriorating relationships, you'll suddenly lose access to what you've been funding, because the US would declare it "a matter of national security" and outlaw the export. You can still work with the sources you have, but it'd be hard to keep it running smoothly. Much better to not have that risk if you're going to spend billions on it.