Well they wanna be the good guys, the keepers of peace ... that's their weakness ... like in Episode 2 ... the big Jedi battle in the arena ... if they would have all used their 'lightening power' that fight would have been over in less than two minutes ... not very good for the movie and for us the viewers
I know, they only use their force and they only resort to violence when needed, to protect their own lives or to protect the lives of those they have to protect
The typical good-guy protector thing... hehe It's a pity they'll be wiped out. Darth Sidious is powerful enough to make his presence undetected, even if they're on the same planet!