R.C. Sproul has spoken out on why he did not and cannot sign the Manhattan Declaration. From his statement:
In answer to the question, “R.C., why didn’t you sign the Manhattan Declaration?” I offer the following answer: The Manhattan Declaration confuses common grace and special grace by combining them. While I would march with the bishop of Rome and an Orthodox prelate to resist the slaughter of innocents in the womb, I could never ground that cobelligerency on the assumption that we share a common faith and a unified understanding of the gospel.
I think that is a great summary. There is more in his full statement that fleshes this out.
I also think it is interesting that a declaration designed to express unity has instead turned into a source of division.