The hair is a rapidly growing organ, possibly among the fastest of body well (outside the intestine). Cancers are also fast growing so when the cancer is treated with Chemotherapy, the treatment targets fast growing cells. That is why many people who get this get stomach and intestinal side effects. Hair loss occurs in the same way. We have written about the Cold Cap to prevent hair loss with chemotherapy and it is an important tool, review posts here: https://baldingblog.com/?s=cold+cap&x=0&y=0 and you can review the many posts on the subject.
Some people only partially regrow their hair if they lost it. This may be caused by permanent changes in the life of the hair follicle which is frequently limited anyway. When a hair follicle reaches the end of its life (from genetic balding or even old age), the cells undergo apoptosis (cell death). The Chemotherapy may damage the hair cycle clock limiting how many cycles the hair goes through. We know that hair goes through aging in all of us and we often see changes of hair character (from a coarser hair to a finer hair) or graying (as we lose the pigment cells that give our hair color.