Because they want to wipe it off the map? The difference between Muslims with nuclear weapons, like in Tehran, is that they are intending them for offensive attacks. Israel has them only for defense. Israel should NOT give in to becoming defenceless until... the real means of defence come into play. In a region where your nation's destruction is a popular goal, you're rather justified to have defence against such evil infidels.
from the article: Obama 'strongly' opposes singling out of Israel at nuclear conference