Because your immune system is weakened due to another affliction. You are therefore more susceptible or prone to candida overgrowth due to a weakened immune system and will continue to experience the ill effects of candida overgrowth until you address the cause of it.
In my case this was and is due to adrenal fatigue.
When I started experiencing brain fog, I searched the internet for answers. The answer I found, although incorrect, was the candida diet.
I never realized it at the time, but I felt better when I was eating carbs and drinking beer than when I went on the full blown candida diet(not a single carb other than what was in vegetables and almonds). On the candida diet is when I felt the worst brain fog, had the worst sleep, and felt the worst fatigue although I did experience some improvements from which I attribute to being gluten free and eating a processed-free diet. As well I quit drinking and tobacco, but this was merely a smoke screen for actual health.
Since then I found a low fat raw vegan diet that gave me the strength to recover far better than anything I had tried. And I am certain that I sound like a broken record when I say I had tried everything, well everything that was supposed to work according to information on the internet.
I do not continue to eat raw vegan, but feel it was a major step in my recovery.
Another thing I believe is that the high fat diet that the candida diet promotes actually exacerbates candida systems due to 1)excessive fat clogging the blood stream, which increasingly causes difficulty for glucose to enter the cells, thus keeping blood Sugar elevated and causing candida to bloom out of control and 2)the restrictive diet forces candida to go systemic and search for food in other areas of the body.
I am not a doctor and can only speak from my personal experience and what has worked for me.