Because its only acts on nodal tissue (i.e SA and AV node). Hence it will not slow down the ventricles since the origin of the arrhythmia is in the ventricles and not the nodal tissue.
Class 2 and class 4 antiarrhythmics only act on nodal tissue.
Class 1 and 3 act on the rest of the heart. If you have read this section from FA you will note that for class 1 and 3 the action potential diagram is of cardiac muscles. But for class 2 and 4 the AP diagram is of nodal tissue.