Help me introspect: Why is there such a skill gap here?

Heya. So, I'm here to share something I've been struggling with and has really brought down my enjoyment of league immensely.

Why am I GOD-AWFUL at champions I don't main? No intent of self promotion here but like, I'll play tryndamere like a gold, maybe yi or urgot like a silver, something like that, if I pick anything else I'll get completely destroyed 100% of the time.

So, why am i enormously worse at non-main champions and roles? Is that just normal?

PS, I don't play ranked.