Why do Supreme Court Justices often grow more liberal but not more conservative??

Thomas Sowell asks some interesting questions on this topic in the Washington Post and posits that:

Although Supreme Court justices have lifetime tenure, precisely to give them independence, nothing can give anyone the backbone and character to stand up to criticism or to resist the blandishments of flattery and lionizing.

All the pressures are to move to the left, in accordance with the views of the liberal media and the liberal professors who dominate the law schools.