There is a general increase in ionisation enthalpy along each series of the transition elements from left to right. This is due to an increase in nuclear charge which accompanies the filling of the inner $d$ orbitals . The variations are relatively small because the added $d$ electrons shield the outermost $ns$ electrons from the increasing nuclear charge somewhat effectively. Thus, both the Assertion and the Reason are correct, and the Reason provides the correct explanation for the Assertion.