Just found this link that backs up what Jorge is saying perfectly.
Resistance to nystatin is very rare, but the problem is that nystatin must come into direct contact with the yeast to be effective. By the time the nystatin gets to your colon from the mouth who knows its effectiveness. Need to deliver it directly where the candida lives from both ends of the body.
"Resistance to nystatin is rare, although the drug's contact killing makes it somewhat more difficult to use because it must be applied to all of the affected mucosal surfaces to be effective (unlike systemic therapies). Failures with nystatin are more common than with fluconazole."