Dua: Supplication to Allah. Invocation.
Salaah: Prayer. The second pillar of Islam. To pray five times daily.
So you have to do mandatory prayers to ask for forgiveness. What happens if you can't do the prayers for a day and you sinned before the day begun and at the end of the day you died still in your sins?
What happens if you sin without knowing about it and since you don't know about it you can't ask for forgiveness. So again you die in your sins.