Electromagnets are made of soft iron because of its high permeability and low retentivity. High permeability means that the core provides complete linkage of magnetic flux of the primary coil to the secondary coil.  The determination of the magnetism left in the material after the removal of the magnetizing field is known as retentivity. The required condition for the operation of electromagnets is the fast reversal of polarity, so high retentivity is undesirable. So, soft iron core has high permeability and low retentivity.