Because of oversight more than likely , but it is canonically the doom guy from doom 1 and 2 and 64 you play as , as the slayer. The dark ages is in fact set between doom 64 and doom 2016 where he starts out in the sarcophagus.
Hugo martin confirmed it in 2020 when eternal came out, feel free to look it up. The outfit in the flashbacks being the classic outfit was likely for impact, even if it was not what he would have been wearing when he closed the portal in 64.
Then again if they remake doom 64, they could use the og doom 1 and 2 armour.