The same reason because Iran has to do it. Rules must apply to everyone. Israel, Iran, Bangladesh, doesn't matter who. No one must be above the law, and this initiative cencerning Israeli nukes was long overdue. If you wish others to be "nuke-free", Israel must be "nuke-free" also.
Sinai bomb attack could have targeted peacekeepers, official says (AP)
from the article: Report: IAEA to discuss Israel's nuclear activities for first time