Probably a combination of amount and length of contract better elsewhere. I don't blame him going for the money. If he were a few years younger ambition might have been a bigger driver, but this is likely to be the last big deal he gets. If he squeezed an extra year out of Leigh then good luck to him.
We'll be weaker without him than with him, but if we're now playing hardball on contracts it will pay off in the long term.