He gave the repentant thief absolution because he asked for it. Notice that the thief who mocked Jesus and told him to save himself did not receive the same assurance.
The problem in the case of someone who is intent on committing suicide is that the priest already knows they are intent on committing a mortal sin (self murder). So even if they gave them last rites, the person seeking death would immediately lose sanctifying grace by the commission of a mortal sin.
The purpose of last rites isn’t to give someone peace. It’s purpose is to prepare a soul for Heaven through the remission of sin. Assuming the Apostolic Pardon is given it also remits temporal punishment. So to what purpose is the remission of sin when someone will knowingly commit another sin?
On top of that, offering last rites to someone committing suicide can be construed as implicitly accepting that it is an acceptable way to end one’s life. The Church grieves for God’s children that take their own lives, but She cannot condone that it is a Godly thing to do.
