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

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.