There is a valid reason for doing so. Islam has always used violence and genocide as a tool for its growth. The existence of pre-Islamic statues remember people about those incidents. Once these monuments are removed, then in future, people will never knew that they had such a rich culture and civilization before Islam.

This is not unique to Islam. Christianity has also used the same. For example, they destroyed the totem poles of British Columbia, as they seemed too "anti-Christian" for them. Christian missionaries have always destroyed native places of worship, in order to force their religion upon others.