Cancer Stem Cells possess drug resistance property not just because of any single mechanism. Drug resistance is a very generalized term used in cancer biology, since different drugs have different mode of action accompanied with differential drug resistance mechanism.
Mainly CSCs can evade the cytotoxic nature of drugs by their high expression of MDRs, efficient DNA repair, Quiescent nature of CSCs along with other cell survival pathways like PIK3CA...
The main property of CSCs which contribute to drug resistance is presence of ABC binding cassette protein ABCG2 due to which these can pump the drugs outside of their membranes and thus resisting the toxicity caused by the drug.
And quiscence is also helpful in reducing/resisting the toxicity as major anticancer drugs work on highly replicating DNA which is property of cancer cells but as CSCs are in a phase of quiscence so they don't devide actively and hence those drugs are not able to act on them.
These two factors mainly contribute to drug resistance in CSCs.