A combination of factors. It could be that the patient's body had adjusted to the previous iui procedures, and the medical team also learned more about the specific situation of the couple through the previous attempts. For instance, they might have adjusted the dosage of medications for hormonal regulation more precisely in the 5th iui based on the experience from previous ones. Also, psychological factors might play a role. If the couple was more relaxed in the 5th attempt, it could have also contributed to the success.
Better patient preparation. For example, the woman may have improved her diet and lifestyle before the 5th iui, which could have positively affected the outcome.