More or less the same reason white cats with multicoloured eyes tend to be deaf...
Simply put, some genetics, or combinations of particular genetics that would result in visible traits also cause the expression of less desirable traits.
The EXACT reason (on a more detailed genetic level) I'm not sure.
Wobble is hit or miss. They all have it to some degree though. In some, it's barely noticeable, in others, it's hard to miss sometimes. Generally it isn't recognized to cause any significant impact on the animal's quality of life (ie. it still feeds, behaves, and functions normally). Some snakes seem to have very little wobble, then all of a sudden, it's there. Other snakes seem to start out with heavy wobble from a younger age. Whether some snakes actually improve over time is something I can't tell you, but would be interested to know more about.
Overall though, "serious" is not the right word to describe it. More like "how noticeable is it".
Edit: Was beat to it.
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