The retentivity and coercive force are smaller for soft iron.

Soft iron is easily magnetized by the magnetic field but only a small magnetization is retained when the field is removed.

The hysteresis loop for soft iron is as shown

The area of hysteresis loop is proportional to thermal energy developed per unit volume. Since soft iron has small area the thermal energy developed is low which is beneficial for using it as a core in the coils for galvanometer.

Therefore, due to low retentivity and low hysteresis loss, soft iron is used as a core in galvanometer.