Why do doctors recommended using minoxidil (5%) for 6 months for newly transplanted hair?

Minoxidil is a non-specific medication that promotes increased nutrition and bloodflow to the roots of the hair. So the benefits are good for newly transplanted grafts.  Minoxidil is not necessary though. Your transplant will grow in fine.

Minoxidil is effective in protecting the non-transplanted hairs by keeping them in anagen phase for longer.  Patients often times need a blueprint for maintaining native hairs so that future procedures are not necessary.  Transplanted hairs have been reported to grow faster with minoxidil.  Evidence has been reported by several studies with limited numbers including at a Consensus Report at the 9th Annual Meeting in 2002.  The main downside of minoxidil is irritation which is often mitigated by the use of foam.  Best Dr. Anil Shah

Minoxidil is an additional treatment that can be helpful to stimulate hair growth both from the newly transplanted hairs and from the existing pre-transplant hairs.  While it can be helpful, it is not required pre or post-transplant.  Lack of minoxidil use will definitely will not cause the transplanted hair to fail or to fall out.  
All the best,