My kid had a cavity that the dentist fixed with a filling now at her six month check up they state we have to pull the tooth due to infection, why?

Because infection: If the baby tooth becomes infected, this can harm the permanent tooth and develop severe infection and pain.

Possible infection: Sometimes cavities can be large or deep and close to the nerve of the tooth. Most dentists will try to restore it with a filling first if it is still not in the nerve. Filling is the most conservative choice. But if after the filling the tooth has a new cavity, or the filling breaks, or the tooth has an infection and is not restorable by other means then extraction may be the only choice left.