Although the general assumption of the public is that supreme court justices are "unbiased," that's completely wrong.
At this point, Supreme Court nominations have devolved from putting real, honest judges into the Supreme Court and more of putting judges with similar ideals as you into the Court.
Presidents tend to look for Justices who have very similar, if not identical, opinions on policies. By doing so, the president assures that the policies he doesn't want will be struck down by that Justice.
Isn't Democracy a beautiful thing?
-T.B.