Shava (dead body) is Shiva! That’s why!
What is a cremation ground? It is nothing bur Shiva’s land (Rudra bhoomi). When we think of worshipping Shiva we automatically think of worshipping Him with waters of Ganga. After offering water oblations, we apply Vibhuti and sandal paste to the Shiva linga. Thereafter a third eye is drawn and it is decorated with Kumkum. It is worshipped with Tulasi and bilva leaves and flowers.
Hindus believe that a dead person has united with Shiva. Therefore it is bathed and decorated on its last journey.