Master Windu was a very powerful Jedi who was comparable to Yoda. Why couldn't Windu's reflexes block Anakin's attack?
The novelisation makes it pretty clear that Windu was too focused on killing Palpatine to worry about what Anakin was up to. He never suspected him of siding with the Emperor in any serious way and he certainly wouldn't have expected Anakin to actually injure him.
“I need him alive!” Skywalker shouted. “I need him to save Padme!”
Mace thought blankly, Why? And moved his lightsaber toward the fallen Chancellor.
Before he could follow through on his stroke, a sudden arc of blue plasma sheared through his wrist and his hand tumbled away with his lightsaber still in it and Palpatine roared back to his feet and lightning speared from the Sith Lord’s hands and without his blade to catch it, the power of Palpatine’s hate struck him full-on.
He had been so intent on Palpatine’s shatterpoint that he’d never thought to look for Anakin’s.
Dark lightning blasted away his universe. He fell forever.