The reasons are, I suspect, geopolitical. Iran has the potential to become a threat in the region within the next five years, be it through military or economical reasons. Nipping that threat in the bud will help to maintain Western influence in the area, well...that's in the best case scenario.
There's also the possibility that Iran itself is fabricating the state of its nuclear program, daring Israel and the West to hit it so that it can claim greater prestige for itself as the 'wounded victim', the 'innocent party' in the attack. Particularly if, after the attack, the IAEA is allowed to look at the sites and find that there's nothing there except 'baby milk factories'.
It's a pretty low blow, but when your enemy has superior firepower, then you have to focus on thinking outside the box in ways to disgrace your enemy, to trick them into falling into traps that you've prepared for them.
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