Why is the Newton's second law called the equation law?

This is because it gives us an equation relating force to . It is so useful that, it is used to furnish a definition of force.

F=dp/dt Implies, F=M.dv/dt (Since, p = Mv, the momentum)

This gives, F=M.a considering the familiar result a = dv/dt

We may also note that, when F = 0, a = 0 , thus it elegantly account's for .

Similarly, it maybe showed using vector algebra that it reproduces the third law as well.