Like, they could’ve just said 蕎麦屋 rather than そば屋 if that restaurant is selling dishes involving soba noodles for example. I’ve seen うなぎ more often when there’s a Kanji for it (鰻) but decide to transcribe it using Kana, same with supermarkets or stores on them writing たまねぎ (玉葱), にんじん (人参), りんご (林檎), まぐろ (鮪). Also stalls that sell takoyaki often write it as たこ焼き (they’ve could used the Kanji for たこ which is 蛸).
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One of these is fucking dense and looks like shit on small signage or from a distance.
The other can be easily stylized, is less dense and looks good on small and large signage.
From an advertising perspective one of these is objectively dogshit the other isn’t. So it’s kinda cut and dry.
It’s like asking why we don’t use cursive on English signage.