"This pertains to a particular historical situation, not to a general condition. The passage appeals to a time when Israelites once had a right relationship with God, when God helped them against their enemies because they waited on him, gladly did right, and remembered his ways."
Several passages in Scripture have a more direct significance and application to the original audience than to readers in later generations. Prophecy has an immediate group of listeners and also a future fulfillment. There are indications which point to Isaiah 64:6 having a universal application.
"When they sin against him and did not repent and return to their former state, he abandoned them to the will of their enemies, so that even Jerusalem and its Temple were destroyed. (Isaiah speaks of this prophetically, before it happened.)"
The sinful state described in Isaiah 64:6 is applied to the entire human race elsewhere in Scripture. In Psalm 14, David describes the pagan world as corrupt and having turned away from the living God. He is clearly speaking of all people. Paul quotes that Psalm in describing the state of Israel (Romans 3:10-18). Every mouth will be silenced as the whole world is held accountable before God (Romans 3:19-20).
"It was during that period of continued sin, leading up to the destruction of Jerusalem in 586 B.C., that they had “become like one who is unclean”–they hadn’t always been like that. In this state, even the nation’s acts of righteousness appeared like filthy rags to God, so he wouldn’t help them: “When you spread out your hands in prayer, I will hide my eyes from you; even if you offer many prayers, I will not listen. Your hands are full of blood; wash and make yourselves clean. Take your evil deeds out of my sight! Stop doing wrong, learn to do right!” (Is 1:15-17)."
Why cannot Isaiah 64:6 apply to every person? It attests to the depths of human depravity and our utter inability to redeem ourselves.