03-19-2015 08:44 AM
ROC curves help you to assess models for a binary target (sensitivity and 1-specificity connect the dots of true positives and false negatives).
Clustering is unsupervised learning, which means you don't need a target.
This is why you don't see ROC curves if you don't have a target that can be assessed in terms of sensitivity and 1-specificity.
I hope it helps,