Because Jiraiya was still alive when he fought him. You might be referring to Kabuto using Edo Tensei. Kabuto wasn't able to revive Jiraiya because DNA has to be obtained from the one you're summoning and Jiraiya's corpse went down the ocean/river/body of water where they fought. It can also be that Pain didn't use Jiraiya's body because the techniques of the 6 paths of pain will only be limited to the one's the current one's have been using NOT Jiraiya's skills himself. Rendering it not really useless? But pointless to just use the body and not his techniques.