1. The opponents know the high tier matchups extremely well since they encounter more of those characters and less of the low tiers.
2. Some high tiers are difficult to play at the levels that truly make them shine. They aren’t like brawl metaknight in this game where any moron could pick him up and figure out how to do semi-decent at him, even bayonetta requires some training and competence to use her effectively when against a decent opponent. So when you don’t fully dedicate yourself to certain high tiers you’ll never be able to play them in the way that makes them so dangerous.
3. Some low tiers have easier learning curves to get decent at them. Granted they’re still low for a reason, and so while some might be easier to pick up than some high tiers, if the opponent knows the matchup and what few options that low tier has and how to exploit them, the low will still likely get wrecked.