Reasons for something occurring doesn't equal "excuses". The things
@Shaqfuey pointed out are valid.
Also, Swipa is not a BAD defender. That's just more hyperbole from fans. I wish some of you would be as strictly and unforgivably critical of your own opinions as you are of these players. But I digress.
We all would like every player on this roster to be better in all aspects. With regard to Fox, we'd all love him to be an all-world defender. Or at least a very good one. But he's not a bad one. IDC what stats you want to cite. Defensive metrics aren't totally objective or accurate. A lot of subjectivity and guesswork goes into them.
Fox is a good defender when he wants to be. Like a lot of players, he conserves his energy and picks his spots because it ain't easy to go all out on both ends. Like most players, he just needs to be more consistent on that end. And develop a little bit more pride in it.