Why did my wife have to wait so long for anesthesiologist to add to her epidural?

Many ideas. May have been that anesthesiologist was busy elsewhere. He may have been at home. May not have been notified by the nurse in a timely manner. I am sorry to hear about the "poor customer service".
Tell me more. What king of surgery? Was this a labor epidural? The staff may have been tied up with another patient. Not enough information to give you a satisfying answer.
Wanted to be brave. Often patients aren't aware that doctors can detect their level of pain and want to help by increasing the epidural and ther will not be any significant side effects.
Labor Epidural? Giving more epidural anesthetics may prolong labor delivery. Other factors, epidural dosage is based on weight, height, age and or presence of existing medical conditions like diabetes, hypertension and other problems. In short, prevention of anesthetic over dosage.
Need more informatio. Not knowing the circumstances of your wife's situation makes it difficult to answer this question. The anesthesiologist may have busy, or was waiting to the see the effects of other intervention. It can take up to 20 minutes for additions or changes to manifest. If the epidural was for labor the anesthesiologist also has to consider was is best for baby and delivery.
Busy. In many places, the anesthesiologist has or responsibilities, including cesarean sections. Also many patients may need epidural at once. Rest assured, every patient gets undivided attention when it is their turn. Most places don't put multiple anesthesiologists on call because it is expensive, and they need rest from long, stressful calls.