It is not like people turn suddenly stupid or suddenly better
It is mainly a psychological question. The fact that rank just exist completely skew the approach of the people to the game, as they feel the urge to carry, especially if they aim to reach the same division or better of a friend or a certain division to get the prestige/rewards of a certain division
Also, especially in low divisions , rank is not a strong meter of the true skill level of a player, as it is not like a true Silver III-IV skill level player on Support position can carry consistently games at Bronze I-II level, for example, the initial placements and being carried or not in those can dictate the rank of such players for seasons as getting in Silver rather than in bronze can end up in having better players around you that can carry, and quite honestly the skill difference is so little that even if you not belong there and you can suck another can easily carry more and lead you to win anyway
I do not play ranked (yet) for a my choice, but I have a friend which, when he actively plays LOL (aka is not caught up by other games es. destiny) complain about one of his friend who is like 1 division better than him even though he is on the same level skill wise if not even probably worse than him at the game...and in all honesty, having seen both of them play together for a number of games and having spectated games of both, he is right at complaining about the difference in their ranks.
There is no way he is sitting in B II-III and the guy is same level/worse than him is in S II-III and that in the past season he got struck by end of season frenzy promotion and didn't get to Silver while the other guy lucked out S I placement, went all way down to Silver ranks till to the point he got demoted like a pair of days before the end of the season on B I, scraped out a lucky last minute promotion to S V while being carried in those games to get Silver rewards by the end of the season, and when placement come one guy get unlucky again and finish in mid-low Bronze and the other one lucked out again and finish like in low S I-high S II, from where he is starting to fall down again despite kep regularly playing
When you see that guys of your level are costantly higher than you because of those reason, and you get angry at your allies making mistakes that cost you games and don't allow you to go up or got angry at enemies getting lucky in trades barely escaping with like 10 hp that subsequently lead you to a loss, and you lose your chill, you start playing worse especially as far as decision making goes, and this generate a chain efect because with your lower level of play you lead other guys to lose, which cause some of them to enter in the same state of mind and so on
In normal there isn't this factor because after all you can get to next game with little consequences on the long term, even though I can still get mad at the time with guys that completely derp around (some of the guys that says "you can go whatever you want in normal" are probably the guys that ruin the most normal games for the others), guys that do over and over the same dumb mistakes without even trying to adjust (and you can see when someone is trying to adjust and when someone is not by stubbornly repeating the same things over and over), or just keep not listening and doing stupid things that lead to loss or lead you to get mad and make you commit mistakes that causes losses or both(and the last occurence happen to me at times, I do stupid mistakes after got mad because people take stupid decisions that are leading us to loss, further worsening the situation).
And also, I would point out, rank should tell you how strong are you in comparisons with the other ranked player, but the population of people that play normal is far superior there are a lot of guys that don't get to ranked because they don't like the environment, don't have good connections/computer and don't want to disconnect during ranked games as it would be frustating or feel they would be at too big at disadvantage because of it and they wouldn't have fun by being at such disadvantage, guys that don't believe to have a large enough champ pool or good enough on a mani champ to compete in ranked, guys which have no time/no interest to keep grinding rank to reach what, mid-high Silver at best given their skill level while still having played a good mount of games, so that your strength compared to the whole population of bronze-silver level player that have never approached ranked cahnge as there are no placements, promotion/demotions or things like those that factor in but just by you should get to your appropriate rank by your sheer playing and performances that in the long term translates in an accurate around 50% W/L ratio at certain level of competition