He said, “Ki am kishei oref hu. Visalachta…for it is a stiff-necked people, and you shall forgive…”
The Ramban, Rabbi Moses Ben Nachman, who lived in the 13th century in Spain, answered this question.
He wrote that the verse means that Moses says to God:
“You know your people. They are a terribly obstinate people. To move them from one level to another, from one position to another, takes a long time. Because of this, you should forgive them.
“You must remember that they lived 400 years amidst an evil nation. You want them to change? You can’t expect them to change in a day. It will take them years and years to change.
“But when they are changed, the same obstinacy will be on your side. They will never leave you.”