Because Old Testament sacrifices indicated reality, but had not the realities themselves.

Hebrews 10:1 "For the law having a shadow of good things to come, and not the very image of the things, ..." (KJV)

These sacrifices provided for in the law were as a shadow evidence of the real substance that was to come. They were merely: "A rude outline (foreshadowing) of good things to come" (Amplified version). In Christ Jesus we have the real reason and once -for-all sacrifice with all the good realities He brings; namely, forgiveness, regeneration, sanctification, life in the Spirit, and ultimately, resurrection. Because Old Testament sacrifices had to be repeated continually, it demonstrated their inability to take away sin.

Hebrews 10:1 "... can never with these same sacrifices, which they offer continually year by year, make those who approach perfect." (NKJV)

"Perfect" - to make a sinner perfect before God would be to have sin totally removed. Because Old Testament sacrifices did not remove the sense of guilt.

Hebrews 10:2 "For then would they not have ceased to be offered? Because that the worshippers once purged should have had no more conscience of sins. " (KJV)

The logical deduction being that if they had been truly cleansed guilt would have been removed and there would have been no need to repeat the sacrifice. Because Old Testament sacrifices were a means of remembrance of sin rather than the removal of them.

Hebrews 10:3 "But in those sacrifices there is a remembrance again made of sins every year." (KJV)

By contrast we are told by the Lord in partaking of the emblems of the new covenant

Luke 22:19 "... this do in remembrance of me." (KJV)

The Israelite came with his sacrifice and remembered his sins - whereas the sacrifice of Jesus, commemorated by us, causes us to remember Him who has forgiven and justified us making us just as though we had never sinned.