From what I remember, Pau had really worn out his welcome in Memphis. The fans were booing him mercilessly. It doesn't help the team make the best trade when other teams can make "take it or leave it" offers.
I remember being furious at the Bulls because they were in talk for Gasol and wouldn't include Deng (they offered Andres Nocioni, Tyrus Thomas, Thabo Sefalosha, Adrian Griffin, Joakim Noah and picks). In hindsight, Memphis would have been stuck with some pretty awful guys and late draft picks, plus Noah. At the time, there was no indication Noah would be as good as he's turned out.