There's a few reasons for me:
First, i played A LOT of normals before starting ranked. My normal mmr is now in the top 4% of players, and it's been as high as the top 1%. I've regularly played against master /challenger players in normals, sometimes on their mains. I legitimately got good in league on my main roles in normals, and switching to ranked it was useless. With silvers feeding, my careful play focused on getting leads through farm didn't work at all. You needed to carry, or you'd lose. It wasn't until i switched roles in to mid lane that i was able to start winning more games.
That's only really a small part though, as I've gained enough experience in ranked now for that to be a non-issue. The bigger reason in the past now, was that i had to write exams for my career twice per year. These exams took about 4 months of studying each. If i came home from work and played a game or two before starting to study, i found that a bad game of ranked (either due to a feeder, a toxic teammate, or my mistakes) would result in me being stuck up on the game and not being able to study at all that evening. I could play normals without worrying about it, but ranked just got in my head. So i didn't play ranked for those 8 months per year, and since one of the exams is the first week of November, of the 4 months i could play ranked, 2 months were off season / pre season. With only 2 months per year to play ranked, there was no way i was climbing to anything meaningful, so i always just got gold and stopped (even though i was regularly playing with people in plat 3-5 so there's little doubt i could have climbed to at least high gold if not plat with some more games).
Of course now I'm done exams, so I'm mostly in ranked. I still go to normals to try to learn new champs - my pool is really small and most of my pool is rather weak right now too, which sucks.