This is not an area of particular knowledge to me, but of the three primary belligerent Arab states involved in the Six Day War (Egypt, Syria, Jordan), Jordan was the only one that had maintained good relations with the West.

This was in large part a Cold War dynamic that generally speaking saw the United States supporting traditional and conservative monarchies like Jordan, Saudi Arabia, Hashemite Iraq, and Iran against the more revolutionary and Soviet-backed states like Egypt, Syria, post-Hashemite Iraq, and South Yemen. (This is a generalization, Nasser was a founder of the Non-Aligned Movement, but was certainly warmer with the ostensibly "anti-imperialist" Soviets after the Egyptian-Czech arms deal and the Suez Crisis from 1955/6.)

American arms sales to Jordan can be viewed in a couple ways. One is the narrow Israeli/Arab conflict lens, in which the US was not at this point as reflexively pro-Israel as the current reputation between those countries would suggest. Various policies, like the 1950 Tripartite Declaration, sought to limit arms sales to the Middle East.

As Michael Oren writes:

While American arms sales to the Middle East multiplied during the Johnson administration, from $44.2 to $995.3 million, Israel’s share was negligible. “The United States had much good will for Israel and desired Israel to have an adequate deterrent,” read a joint memorandum of February 1965, but Johnson refused to be Israel’s primary arms’ supplier.57 That refusal reflected America’s traditional reluctance to identify itself totally with one side in the Arab-Israeli conflict or to get entrapped in a Middle East arms race.

To maintain that balance, tit-for-tat arms sales were carried out, so Oren again:

"American M-48 Patton tanks had been sold to Israel— albeit indirectly, through Germany, with a counterbalancing tank sale to Jordan"

In the second and wider Cold War lens, Jordan was a natural US ally against communism, and weapons sales were (and continue to be) a primary tool of US foreign policy to ensure the support and stability of allies.

Furthermore, failure to provide those weapons to Jordan might well have meant a Jordanian deal with the Soviets. From a 1965 NSC memo:

"Since the Israelis seem to have focussed only on the added threat to them from US arms sales to Jordan, you should lay heavy emphasis on the far greater threat which would ensue from uncontrolled Soviet or [United Arab Republic] supply. You should point out that from our talks with King Hussein we are convinced that, unless he can get minimum satisfaction from us or Western European sources, he will be compelled to take the Soviet route."

So combining these two, given the US interest in not particularly favoring one side or another and keeping friendly Arab states on side, it's unsurprising that Jordan got that US support while Egypt or Syria did not.

As for whether or not that continued, I'm not sure about the specifics of what deals were signed when, but Jordan certainly continued to be a key US ally in the region. Here, for instance, is a message from Ronald Reagan about arms sales to Jordan in 1985.

That being said the "uniqueness" of Jordan's relationship with the west in the context of Israeli-Arab relations was significantly diminished following Egypt's 1979 peace agreement with Israel, which included massive US pledges for military aid and arms sales. Egypt was named alongside Israel in 1989 as a "Major non-NATO" US ally, while Jordan was added to the list in 1996 following its own peace with Israel in 1994.