i do not know for certain. here is my thought:
white blood cells fight infection.
if his white count gets very low (<0.5), even the smallest infection has the potential to cause sepsis (infection everywhere) and kill him. they do not want to see him die.
if they give him chemo, his white count drops very low. Then they postpone giving him the next round of chemo until his white count is back to normal. Depending on how long that takes, he may not be getting enough chemotherapy to have much effect on the cancer.
As I said, this is just my first thought.
Perhaps contact the doctor running the study and ask him why, and also ask him for suggestions as to what to do next. This may be best done by making an appointment to see him in the office and sitting down with him face-to-face with your husband's records in front of him and talking.