In any event, it isn’t because she covered her hair, as the gemara says ???? ??? ?? ??? ???? ????.
This seemed wrong to me, as they asked her (they obviously valued her opinion), and the Gemara records her answer. I found an Ohr Zarua who says that she had another z’chus as well, but certainly her tznius was part of it.
http://www.hebrewbooks.org/pdfpager.aspx?req=8726&st=&pgnum=8
?”? at the end.