"Why hasn't the U.S. pressured Israel?"
The default answer on "why didn't X do Y?" is "Because X didn't have a reason to do Y". Not doing something is easy. Hence, we should look at the question to see if there are any reasons for the US to do so.
"45 member states, including Turkey, Russia, China and Iran, back the proposal from 18 Arab countries." On the whole, having those countries on the proposal would be a reason for the US to reject the proposal. Those aren't the historical friends and allies of the US.
"Israel has also criticized and condemned Iran for their nuclear program". So has the US. In other words, this finds the US and Israel agreeing, so it can't be a motive for the US to put pressure on Israel.
Now, we might consider some hypothetical reasons why the US might want to act like this. Maybe the US would believe that the Middle East would be more stable if nobody had nuclear weapons. On the other hand, Israel's nuclear capability probably explains why they haven't been at war with their neighbours for a while. If Israel would give up their nuclear weapons, the US probably must intervene to save Israel in the next war. Because as the question notes, those Arab neighbours still don't recognise the legitimacy of Israel.
In the end, we have to conclude that there are no strong objective reasons for the US to act, which is consistent with the observed lack of action.