Nuclear DNA is inherited from all ancestors. But mitochondrial DNA is inherited from a single lineage. The DNA inside of mitochondria (called mtDNA) unlike DNA inside the nucleus, comes from only mothers.
It is not a mixture of our parent’s DNA, and each person’s mtDNA is nearly identical to his or her mother’s mtDNA.
Thus only nDNA being more unique is more useful for identifying in forensics. In cases where nDNA is not sufficient, mtDNA is used.