It's much easier to intercept said nuclear weapon because instead of minutes you would have months to do it. And attacking Mars outside the few months every two years that have the most favorable transfer windows would also be much more difficult. It's a non-trivial advantage.
Of course, if you have nuclear weapons already on Mars that can be remotely triggered from Earth this doesn't apply, but hopefully we can avoid that...
TBH I think this whole discussion - that fixing Mars should be easier than fixing Earth - is some proof that a lot of hacker news commenters are already aliens from Mars.
Of course, if you have nuclear weapons already on Mars that can be remotely triggered from Earth this doesn't apply, but hopefully we can avoid that...