It's not where they fall on the ideological spectrum, it's their ability to get five votes. Chuck Schumer in the NY Times:
Senator Charles E. Schumer of New York, a Democratic leader who shepherded Justice Sotomayor through confirmation, said the focus should be finding a nominee who could influence Justice Anthony M. Kennedy, the swing vote on the closely divided nine-member court, rather than selecting a firebrand who writes powerful dissents.
“One of the most important qualities for the new justice is the ability to win over Justice Kennedy,” Mr. Schumer said. In other words, he added, “somebody who’s going to be one of the five and not one of the four.”