In his post on why a President or Governor might select a judicial nominee who has no prior judicial experience, Josh suggests that it may be because you can get better insights into the non-judge's views of the law based on the cases they have litigated as a lawyer:

One possible reason is that lawyers get to pick their cases. By contrast, lower court judges can only decide the cases that come before them, and in most cases, are constrained by stare decisis. Their opinions are not a reflection of their personal views, but--or at least should--follow the law. Advocates, especially those who engage in strategic litigation, are differently situated. They can advocate for their bona fide conception of the law, and are not subject to the same constraints. In many regards, non-judicial experien

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