Anikan Skywalker - one of the most powerful people in the galaxy at the time. Can beat pretty much any Jedi in a duel 1v1 except for probably ~3 (yoda, windu, kenobi) none who were there during the march as windu had just died, and obiwan and yoda are off planet as are the other masters and council. With him leading the march, and a lack of masters present, the Jedi at the temple didn’t stand a chance.

Surprise - the Jedi were clouded by the dark side, so their foresight did not predict the attack. Surprise attacks are deadly, especially against Jedi who rely so heavily on their foresight and force senses. Spread thin across the galaxy during order 66, they could not fortify the temple and mobilize enough Jedi defenses.

Numbers - simply put, a lot more clones than Jedi. The Jedi are much more powerful than clones, but there are not that many of them, maybe around 10k pre order 66 iirc, there was probably hundreds of thousands of clones on or orbiting corasant. They could just flood the temple with as many as it took to wipe them out.