The small bowel is a hard environment for microbial growth. It is technically almost sterile since the HCL, Protease, Bile, Sodium Bicarbonate keep it clean. Candida colonies growth better where fermentation, putrefaction, and decomposition take place. The colon is where the biggest fungal concentration is. Most antifungals we take orally go to the blood having almost no impact in the colon. Nystatin, Natamycin, and oral Amp B are poorly-absorbed antifungals but their effect in the lower intestine is weak. The antifungals that may reach the colon do it mixed with dead flora, feces, and wastes.

Taking the poison directly to where the fungus is gives fast results. Nystatin retention enemas after cleaning the colon with water enemas speed the recovery more than anything. The fungus die when enough Nystatin concentration reach the colon walls.
I hope this help.
Jorge.