Forensic genetics- why is mtDNA comparison sometimes better than nDNA comparison?
mtDNA is present in a much higher copy number per cell than nuclear DNA. 

According to this paper, there are approximately 4000 or so mitochondrial DNA copies per human muscle cell. 


  copy number of mtDNA per diploid nuclear genome in myocardium was 6970 ± 920, significantly higher than that in skeletal muscle, 3650 ± 620 (P = 0.006).


This makes it far more likely that a non-degraded copy of mtDNA exists, considering that there are normally only 2 or 4 copies of nuclear DNA per cell per locus for almost all kinds of cells.