When does this happen? Did you start from the beginning, or from a later chapter?
It's because Isaac can no longer be classified as an audience self-insert. Arguably, he never was. The original intent was to let players fully immerse themselves in his situation by letting them choose his name both pre and post Transition, but since he's got a defined personality, backstory and storyline, it's far better to give him a clearly defined pre-Transition name also.