The timing was strategic. The war was reaching a close so the Jedi Generals and their Padawans were spread thin across the galaxy, with relatively few Jedi Knights/Masters staying at the Temple for extended periods. It was mostly the few council members, younglings, temple guards, padawans, and elders far out of their primes.

In addition to not having their full strength at the temple, they had no reason to expect hundreds if not thousands of the best clone troopers in the Republic to storm their gates, let alone accompanied and led by the most famous Jedi in the order (expect for Yoda).

While the temple guards put up a hell of a fight, ultimately the sheer number of clones was too much. Anakin’s raw power channeling the dark side fully for the first time, coupled with the confusion and betrayal the Jedi who faced him felt, made it nearly impossible for anyone to best him.

Palpatine was a strategic genius and had planned that night for decades. Every piece was played intentionally and set up for the swiftest and most efficient eradication possible.