From what I looked up, there isn't any definite answer for it. Just speculations, like a possible copyright infraction. instagram.com/p/99kzwtl_gR/?hl=en
Haine oh no. I thought I could one day listen to Arthur singing. Well but if Sakurai san took this decision then we should respect it. Regardless I just really DW type moon to release fate prototype so that we can hear more of Sakurai San talking as the Prince Charming he is. King Arthur is perfect role for him.
Could you do Megumi Ogata? I'm pretty sure she has quite a few character songs. Tho the only one I really know is Absolute Hope Birthday for Nagito Komaeda from Danganronpa 3.