It's a leftover of an earlier design seen in the July 1993 issues of PC Engine Fan and Gekkan PC Engine. This also carries over to Maria's sprites.

I've never liked this sprite design much tbh. It really looks nothing like the way he is depicted in either the ROB cutscenes / artwork or especially SOTN portraits. Even when they updated the outfit for the saturn port of the latter he's still left with the same stupid blue coloured mullet. They already had a good frame of reference to work from a sprite version of the Kojima design with the correct hair that appears briefly during one of the early dialogue exchanges Between Alucard and Maria Renard, so why not just build on that? The inconsistency irritates me.

So naturally it ended up being one of the most reused character assets across the whole series.