Why did jesus said it is finished

Jesus became the final and ultimate sacrifice for our sin. The word in this verse, “finished,” is actually from the Greek word, “tetelestai,” which is the same word that means “paid in full.” Often, it was used in an accounting term, which indicates a debt was paid. The uniqueness about the way it was written is that the tense of the word indicates both a point in time it was complete and that it would also continue to be complete or finished. And this is the essence of what Christ came to do. He came to “finish” God’s work of salvation in us. He came to “pay it in full,” the entire penalty, or debt, for our sins. He’s at work in our world still today in powerful ways.

The ESV Study Bible comments,
“It is finished proclaims that all the work the Father had sent him to accomplish (cf. 4:34; 9:4) was now completed, particularly his work of bearing the penalty for sins. This means there was no more penalty left to be paid for sins, for all Jesus’ suffering was ‘finished’ (see Heb. 1:3; 9:11-12, 25-28).”