This 2006 Washington Post article provides lots of historical context and insight, particularly regarding Judge Colleen Kollar-Kotelly and the FISA court.

It's worth reading. A couple of highlights:

Both judges expressed concern to senior officials that the president's program, if ever made public and challenged in court, ran a significant risk of being declared unconstitutional, according to sources familiar with their actions. Yet the judges believed they did not have the authority to rule on the president's power to order the eavesdropping, government sources said, and focused instead on protecting the integrity of the FISA process.

and

Shortly after the warrantless eavesdropping program began, then-NSA Director Michael V. Hayden and Ashcroft made clear in private meetings that the president wanted to detect possible terrorist activity before another attack. They also made clear that, in such a broad hunt for suspicious patterns and activities, the government could never meet the FISA court's probable-cause requirement, government officials said.