Michael Froomkin (Miami) sums up the reasons here. Most of the cases I've heard about were unmitigated disasters (for one or all of the reasons Professor Froomkin notes), which is why the most elite law schools never choose them.
(An exception that may prove the rule is former Federal District Court Judge David Levi, who took over as Dean at Duke not long ago--but he, of course, has an unusually academic pedigree (among other things, he grew up in Hyde Park, where his father, Edward Levi, was Dean of the Law School and eventually President of the University).)
UPDATE: Professor Geoffrey Corn (South Texas) writes:
I wonder if you have considered the experience of retired JAG officers as Deans? Walt Huffman at Texas Tech seems to be quite successful, as does Jim Rosenblatt at Mississippi College School of Law, and John Hutson at Franklin Pierce. I think the jury is still out on Tom Romig at Washburn. But I think this is an interesting take on "practitioners" as Deans.
I don't have enough information about these cases, but Professor Corn may well be right that Deans with this professional background may present a different case.