Why do both the north and south poles of a bar magnet attract a piece of soft iron?

Answer 1

The magnetic domains in soft iron are easily oriented by a surrounding magnetic field, so that the piece of soft iron becomes a temporary magnet (by induction).

If you approach a N pole, the domains in the soft iron will orient themselves with their individual S poles towards the N pole of the permanent magnet. Thus, the entire piece of soft iron is 'induced' into being a magnet, presenting its S pole to the permanent magnet's N pole = attraction.

If you approach a S pole, ..., presenting its N pole to the permanent magnet's S pole = attraction.

Answer 2

because iron is a substance which is capable of getting magnetized