• It's because the answer has a net score of $\leq -3$ (though it might happen at net scores of $\leq -2$; I'm not altogether sure when the "graying out" kicks in.). When an answer is heavily downvoted, it "grays-out", unless you hover over it. This is by design, to help deflect from a potentially problematic answer. – Namaste Nov 14 '18 at 22:01
Indeed, an answer appears in gray text (unless hovering over it) when its net score is $$\leq -3$$. Please see this post on Meta.SE for confirmation.