First, because once something bad goes on your credit, it must stay for at least 7 years. If it ruins, or even hurts, your credit, it has to to keep hurting until it is 7 or 7 1/2 years old, even if you pay earlier.
Second, because you have been cash only for the last 5 years.
The damage from whatever ruined your credit cannot be repaired when it is only 5 years old, even if it has been paid.
And being cash only means there is nothing good going onto your credit.