I would guess it is because the clamping part of that stem is likely going to be below the expander bung within the steerer tube. You can however get longer expander bungs which would solve this issue.
I figured that might be the case. I've got a couple of those long expanders in the cellar so might just give it a go.
@atk started
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I would guess it is because the clamping part of that stem is likely going to be below the expander bung within the steerer tube. You can however get longer expander bungs which would solve this issue.