The UN, nor the US, was "terrified" of just Nasser. When considering the role of the world community, you have to factor in the Cold War. If the US/UN *had* stood up to Nasser, it may have only led him to jump further into bed with the Soviet Union. Compare the victory of 1967 with the near-disaster of 1973, when a Soviet equipped and militarily indoctrinated Egypt put the Israelis in the greatest peril of their nations history. If the Arabs had not attacked in 1967, then 1973 may have come as an even greater surprise to an even more unprepared Israel.
The Western nations feared not just Nasser, but Nasser backed into a corner and going to the Soviets hat-in-hand. In particular, while in retrospect Nasser's "pan-Arab" goals were more dream than achievable, there was a very real fear that with Soviet backing he could actually make pan-Arabism work. Recall that well before 1967 Nasser was being decried as a "Hitler" and was the subject of "destabilization" efforts. An international naval armady may have sounded like a good idea, but what if it hadn't worked? There woud have been one thing worse than ignoring Nasser, in the West's opinion, and that would have been putting up a "show of force" and then having it not work. Even worse, it could have been a US ship rather than the Israeli "Eliath" who first demonstrated the danger of anti-ship missiles.
In the end, of course, it is not like Israel was "abandoned." Far from it, the US gave every kind of support just short of actual military intervention. We were in a poor position to intervene in such a manner, both politically and militarily. In a perfect world than maybe the forces of right can always come charging in on a white stallion. In our world, however, things are rarely that simple. The assumption here seems to be that if we "stood up" to Nasser, he would have just slunk away with his tail between his legs. But what if he had called our bluff, instead? By 1990/91, after the 'Carter Doctrine', with no Vietnam in progress, and no Cold War to hamper us --- we were ready and able to put Saddam Hussein in his place. In 1967 we were in no such position, and for us to have pretended otherwise may have resulted in even greater tragedy.