If you go as far as that, why wouldn't you test mitochondrial DNA? Using Ocamm's razor myself, I will speculate that she wasn't able to get a very good lawyer.
1. Mitochondrial DNA is less apt than regular DNA to accurately identify motherhood since it has a low mutation rate.
2. The mitochondria would still come from the chimeric cells which still are a generation away from the mother's cells.
3. If you have have considered the possibility of chimerism, there are much simpler tests - e.g. the tests that they did do.
And isn't testing for mitochondrial DNA fairly simple/cheap?
Even if it's now possible/cheap, that's a recent development.
mtDNA is good for finding your (maternal) ancestral group, less so for your immediate relatives.