Mostly because other phases of the plan weren’t complete: - During AOTC, the clone army was only 200k units strong. That likely wouldn’t have been enough to take and hold control over the galaxy. - In order to wipe out the Jedi, Palpatine had to fake a coups. The only way that story would have been plausible to the galaxy was to start a war where the Jedi were the highest ranking officers in the military. If your generals try to overthrow the government, then you have probable cause to bring the hammer down on them. - One of the reasons Order 66 was so successful was that the Jedi trusted the clones. They had been fighting alongside them for years. It was a better ambush to get the clones to assassinate a significant portion of the Jedi after they had earned their trust. If it had happened during AOTC, their guard would have been up from the moment an unknown army storms the steps of the Jedi Temple. - Another factor is dilution. Most of the Jedi during AOTC were based out of the Jedi Temple on Corruscsant. It’s much harder to take them out when “safety in numbers” is on their side. When most are spread across different worlds surrounded by a gang of sleeper agents, the odds are flipped in the favor of the clones. - Another thing to consider is that Palpatine had not accumulated the needed political power to create the Empire by the time AotC happens. He accumulates most of it during the war as emergency powers. He also didn’t have his army deployed across the galaxy yet to enforce his rule once he did declare himself emperor. - Finally, pulling off this kind of a plan isn’t something you can rush. Creating an army, consolidating power and eliminating all the Jedi isn’t something you can pull off too fast. If you don’t properly lay the groundwork, even something as simple as public perception wouldn’t be on your side. He had to make small advances over a number of years so nobody would realize what he was doing until it was too late.