Lawyers who work on a contingency basis take their fees from money their clients recover as damages; if the clients collect nothing, the lawyers get nothing. Defendants in criminal cases don’t recover money damages if they win, so there’s no pot of money from which an attorney can collect fees. Furthermore, while contingency fees are common in some types of civil cases (particularly personal injury cases), contingency fees are considered unethical and are not permitted in criminal cases (Rule 1.5(d) of the ABA Model Rules of Professional Conduct).