Bowser used to have 10% armour on literally everything and several characters like marth could just never get him into hitstun and he was nigh uncomboable. Even though 6% is a big difference from 10% it would still bring up problems where characters like Pikachu and ZSS can't touch him.
The funny part was that a lot of those characters were even with or beat him anyway, because of the ridiculous mobility they had and his susceptibility to grabs.
6% would leave Pikachu and ZSS totally able to hit him - Pikachu has SHAC dair back, not to mention nair, while ZSS has her normal aerials and ftilt, not to mention side+b.
Old Bowser did not have the 10% armor when in hitstun, so if you did get him into hitstun you could have comboed him. It was still awful design of course.
I talked to an original dev team member who's no longer with the team [w0n't drop names] and he told me they actually built Bowser first, asking the question, "How can we make a traditionally awful character viable in a game where everyone has even easier combos and he's still a giant target?"
I would assume you knew this [unless you weren't an original team member and were never told this], and I'm not saying they met the goal [I'd have to play the original build but there were probably characters who just bodied him even with the armor], but the original reasoning made sense [if not normally, than it made Minus sense], but it meant some characters had to play matchups completely differently from how they fight every other character.
I still actually want to try that build and ideally find someone who can play Bowser well and fight them with Fox [since everyone on here seems to cite that matchups as terrible, which this original dev disagreed with], but I haven't gotten around to it yet.