Because the string regenerates itself readily in youth, but for whatever reason ceases to do so in old age.
But does this string regeneration itself carry a limited life? Perhaps hard to discuss this without talking about integrals of strings... I guess a better question might be: Is it extrinsic or intrinsic factors that prevent string regeneration? Are the oldie stem cells old because they were damaged? Or because they were pre-programmed to wind their division down over the years? My stem cell knowledge is poor =(Because the string regenerates itself readily in youth, but for whatever reason ceases to do so in old age.
The hope, and my hypothesis, is that the young marrow can partially replace and possibly recapitulate functionality lost over time. The marrow contains stem cells for multiple systems, including blood and bone, so the thinking is that these cells can create a genotypically, and hopefully phenotypically, younger progeny. Transplanted marrow cells can expand and replace after ablation from chemotherapy, so the potential is there. Since it's your own cells, your body shouldn't reject them, and hopefully, they will slowly replace the older cells. Of course, the body might signal them to do otherwise, but to the extent that it doesn't, there may be benefit. I call it 'temporal transplantation therapy'. :)