Why do I have to stabilize my hair loss before HT procedure ?

Normal hair density allows for about half the hairs to be taken from a donor area without it being obvious that they were removed. If your hair loss is rapid or unstable, it can be difficult to know how many donor hairs are appropriate to take for grafting. If you have future hair loss, you could end up with uneven density or areas of non-coverage. Hair loss can continue if the cause is not addressed. The most common cause of hair loss is male pattern or androgenic loss. This occurs because DHT, a breakdown product of testosterone, causes hair follicles to shrink. Propecia/finasteride blocks formation of DHT so further hair follicle damage is stopped. The hairs on the top and sides of the head are more sensitive to DHT related hair loss, so as long as there are any non-grafted hairs left in those areas, medications need to be continued to maintain results.

Every patient is different and every doctor may have different opinions.  What you "hear" on the Internet may not apply to you.  

Hair loss is a progressive condition and thus needs to be stabilized on a continual basis.Hair transplants replace some of the hair that has been lost but untransplanted hair still needs to be maintained for best results.This can be accomplished with adjuncts such as Minoxidil and/or Finasteride.

Unfortunately, genetic pattern hair loss is progressive. A hair transplant only works by moving healthy hair follicles from one area of the scalp to another, but it does not, in itself, halt the progression of the hair loss in the follicles that have already been affected. Without a good hair loss prevention program, which typically includes medications like minoxidil or finasteride but could also include other hair restoration treatments like platelet rich plasma or red light laser therapy, the pattern of thinning and loss will almost certainly continue. Although the follicles that were transplanted during the procedure should continue to grow normally, the older follicles that surround them will not, so a patient could easily end up losing the same number of hairs that were transplanted, and possibly even more, in other areas of the scalp.
During the very first consultation with any new patient, we stress that every transplant patient should undergo a preventative program before, during, and after their hair transplantation surgery. Patients who fail to follow the maintenance plan that we establish together prior to their surgery, who do not attend follow-up visits, or who refuse to take all medications exactly as prescribed, will most likely find that their subsequent hair loss eventually undoes any progress that we might have made, leaving them looking worse than they did before their hair transplant procedure.

I am assuming that by "stabilization" you are referring to the use of things such as laser, minoxidil and finasteride.  These non-surgical adjuncts help prevent further hair loss and improve function of the native follicles.  If you want to have good, long-lasting results it only makes sense to do everything to keep the hair you still have in addition of having a transplant done.

stabilizing hair loss has value to reduce the reactive hair loss that frequently follows hair transplants, see below