My first thought would be power usage -- 5700X is 65W, while the 5800X & 5800X3D was 105W. However, that turns out to not be true, as a quick check of the 5600X3D has it rated at 105W, while the 5600X was 65W. So that's not it.
I suspect the real point is probably just yields, the same reason the 5600X3D existed (and maybe the 5700X itself, iirc). These may just be the same chips, with 8 working cores (vs 6 for the 5600X3d), but clocked lower.
It's not even in the comments, it's in TFA: