Question

First order logic (FOL) is called `first order’ because

Answer 1

The First-order logic (FOL) derives from the "first in the logics" involving quantifiers.  

Explanation:

The reason why FOL is considered as first order is because the first-order predicate is made. The predicates are first-order relations in the FOL.

It also has different names like "first-order functional calculus". Moreover, it is merely a set of function symbols and even a set of the predicate. Thus, a predicate can only be stated as to a "single subject" in FOL.