Why does it keep two surnames for me?

Because each one connects you to a different family. Your birth surname is what connects you to the family you were born into, and your current surname is what connects you to the family you married into. Losing either loses a real relationship.

Your birth surname also decides which family line you belong to on the tree — other relatives are connected to you through it — which is why it is set when you join and cannot be changed here. Your current surname follows marriage rather than a form, and it is shown wherever it matters.