Because Israel is not a signator of the NPT, while Iran and Syria are. If Israel ever does sign the NPT, it will be as a country with a declared nuclear capability--the NPT is not retroactive. All Israel's signing the NPT now would mean is that it agrees not to make any more nuclear weapons. Since all the "expert evaluations" of Israel's nuclear capability are filled with expressions like "estimated to have..."; "it is believed that..."; "interpolations of reactor capability indicate...", it's clear that these statements are nothing more than "guesstimates" that may or may not be true or right. I'm reminded of the joke that a black stand-up comic, Dick Gregory, told in the 60s in front of an audience in New York, full of southerners who didn't laugh at his topical jokes about segregation, integration and discrimination, until he asked, "Wouldn't it be one hell of a joke if this (pointing to his black skin) was make-up, and you folks were being tolerant for nothing?" Well, wouldn't it be a joke on the Arabs if there were no Israeli nuclear weapons, and they were being fearful for nothing?
20 killed, dozens wounded in bombings in Sanaa, Yemen (Reuters)
from the article: Syria's threats and counter threats