Why didn't Rehnquist resign months ago?
The official line is that Rehnquist needed to work to keep his mind off of the cancer treatments.
I believe he didn't resign because it would have forced Bush to have two nominees under consideration at the same time. In that case, Bush would have been pressured to provide a liberal and a conservative nominee. This pressure would have come from democrats, the MSM and media-whore RINOs like McCain. Rehnquist despised the MSM as much as any Freeper and was undoubtedly well aware of how they would react to having two nominees under consideration at the same time.