Cause most Jedi do not believe in cheap shotting people. It is one of their many flaws. Also I think they should have just blown the place to bits. So you lose 2 jedi, 1 senater and a bunch of enemies. But you prevent a galactic war. The jedi however underestimated the situation and what Mace was doing in shatterpoint was kicking himself for making that mistake. Also more proof that Jedi make bad Generals is how easily they can be killed by their own troops.