I have not read the entire manga, but I read that Soma's father was formerly known as **Saiba Jōichirō**. It is not the same last name as Yukihira Soma, so this may be a reason why people don't know that they are father and son.

Source: [Jōichirō Yukihira](http://shokugekinosoma.wikia.com/wiki/J%C5%8Dichir%C5%8D_Yukihira) on Shokugeki no Souma wiki.