I was raised in a religion which doesn't believe in cremation. So, to members of that faith, burning your loved one can be viewed as sinful and offensive and incredibly distasteful.
His dumping them in the oil vats does not automatically equal his guilt of murdering all of them, nor does it make him automatically never having been a loving father. I get that it's hideous and distasteful, however, it really isn't an unarguable "be all end all" fact in proving he's the killer of all of them.
It could have been symbolic burial in a place where he knew where they'd be, they'd be "protected" from elements and animals. If he was so worried about animals giving away the location, perhaps he wouldn't have left Shannan in a shallow grave. I don't know, but I don't claim to know exactly what that means. Only Chris Watts knows exactly what he was thinking when he disposed of his children in that manner and location.