In ALttP, the maidens say that the blood of the Hylia is thinning and they are losing their powers. My theory is that, as the blood of the Hylia mixes with that of common folk, successive generations have less and less Hylian powers. By the time of Oracle, the descendants of most of the original Hylians have so much human blood in them that they have become completely normal. That is why Malon, Talon, Ingo, etc., have normal ears - they have become normal humans. After ALttP, the only pure Hylians left in Hyrule are the Hylian Royal Family, the Impa family, and Link.