Alternatively: Powell's views on abortion circa 2000 aren't much different than George H.W. Bush's circa 1978 -- and are clearly to the right of Mitt Romney's circa 2004. One could easily imagine him "modifying" his views if he truly wanted to win the nomination.
That seems like the exact reason why Powell wouldn't want to run. He wouldn't want to "modify". The problem with being known as the independent man of principle is that "modifying" isn't in your playbook when it comes to issues like that.