Perhaps because the US hadn't supplied massive bunker bombs and given permission for possiblly thousands dead in collateral damage? Perhaps because timing is also important?

Otherwise it's self evident that there would be eyes on every movement. It can't be disputed that as above there's an entire anti Hezbollah constituency in Lebanon...not indeed another in the anti Assad Syrian refugees. Then there's the US / UK network of infiltration of the Lebanese military who also run various remote surveillance operations from Cyprus.

Perhaps you mean "Why now"?