You have to remember that no-one suspects Palpatine will turn evil and crush the Republic and build the Empire, except maybe Anakin. Quote:
Palpatine: You don't need guidance Anakin. I can see your becoming the greatest of all Jedi.
All of the good guys believe Palpatine is honest and trustworthy, so it must take quite a bit of time before the Rebel Alliance is built up. Not sure we'll see it in Ep III.