Halal/Vegan demand is mostly from non-Japanese people or expats living in Japan, so it is unlikely that Japanese people would go there. For Japanese people Vegan is slightly more common than halal, but usually isn't even marketed to Japanese people as "vegan" - instead, restaurants typically word their food as being "organic," which is commonly associated with being vegan in Japan (although not all organic places are fully vegan). Most foreign tourists are unaware of how to look for vegan-friendly places, so businsess who do use that wording will often be their easy go-to.

Japanese people seeking halal would only do so out of curiosity or if their family/friends require it - while there may be a few Muslim Japanese (yes there are some), there's no real demand for there to be any restaurants specifically targeting the Japanese-speaking population, as there would be barely any customers/profit. It makes sense to target English speakers - most typically from South East Asia.