Why is my regeneration serum maxed out at a level of 75?

There is a limit to the amount of serum that will be given to you. The game will gift you up to 20 a day until you reach a cap. I think the cap is 60 by default. During the Reign of Chaos event you can purchase the package that will increase this to 300. You can still earn more serum than the cap from the event rewards and killing the Wight king enemies, but you won't get the free 20 a day while you are above the cap.