I think it was because he debuted before B.A.P as Bang Yongguk with I remember. While it is his real name, the full version is his stage name as well.
Pretty much what @anna-something said. When he released I Remember he was already going by Bang Yongguk, so it had most likely been established as his stage name from debut
Right! *smacks self* Obviously. I totally forgot he was Bang Yongguk between Jepp Blackman and B.A.P. It’s only natural he kept that stage name.