There are many strains of candida. The most common is candida albicans, but others are nearly as common in humans with candida problems.
Nystatin only kills some of those strains. If you have another strain in you, then the nystatin killing off only one of them would leave "open turf" for the other strain(s) to grow more.
And you'd end up still having candida problems, and now they won't respond to nystatin.
However, I do not feel that this means no Rx's. I have been told that people with very long standing candida problems (more than a year) may take years to get well on only herbal products with a very strict diet, and maybe not even then, and that if they find an Rx that does kill whatever strain of candida they have, then healing goes much faster.
I have also been told that it is NOT that candida "becomes resistant" to an Rx, it is instead that if there are more than one strain living in you, and the Rx you use only kills one of those strains, then you are stuck with the other strain.