Because it'd be totally weird for someone with otherwise high intelligence to "just be bad" at something basically everyone else with normal+ intelligence is good at. Like, you wouldn't expect there to be an explanation? What does "just" even mean here?

He can, but if we ask why and we find out it's for the same reason a bunch of other people are bad at spelling and the common name is 'dyslexia' then maybe that's a good word to explain why he's bad at spelling?

Being naturally bad at something isn't magic, there is a reason and mechanism

Same as being naturally good at something, there is a mechanism