Why does calf still have diarrhea nursing on cow?

Diarrhea, or scours, occurs if there's a digestive upset the calf is experiencing, such as from bacterial or viral infection or coccidial infection. Anything, really, can cause a calf to scour, whether it's environmental stressers or parasites or, as mentioned, exposure to bacteria in the gut. Either way, the calf will need to be given electrolytes to prevent dehydration--even if he's on milk--and quarantined so that if he has an infection that is contagious to other calves, he won't give it to them either. You will have to contact your vet so that some tests can be done to see what's making him scour, why and what can be done about it.