Why did Saul's sins lead to his rejection as King of Israel, but David's sins did not cost him his crown?

I Samuel 15;10-12 tells us "The word of the Lord came to Samuel, saying, 'I greatly regret that I have set up Saul as King, for he has turned back from following Me, and has not performed my commandments'" The New Testament tells us in Romans 11:29 that the gifts and calling of God are irrevocable. Is that a privilege only given to those under the New Covenant? King David sinned by committing adultery and murder, yet it didn't cost him his crown; Saul's sin was to offer a sacrifice when it wasn't his place to, a sin of presumption. Was this a greater sin?