i always forget that i play isaac on hard mode. i think most isaac players do, it is really "normal" mode in that sense.
majority of the unlocks are locked behind beating objectives on hard mode, so there really isnt a reason to not play on it. it feels like weird design to me that ive honestly never really thought about. like if someone was starting now id almost consider saying, just play on hard from the jump as you'll never not going forward. doesnt feel right to say that tho
(hard is less consumables, more champion mobs, and slightly more aggro enemies)