Losing 1 ranked game to be placed in Iron league

Hi! So I played a ranked flex game and I lost. I was not able to win because a lot of lanes lost. still managed to get a A+ and I would argue I had a lot of positive impact on the game. but! after this 1 game I instantly get placed in Iron league. What exactly is the reasoning behind this? I played it solo, and in solo/duo queue I got myself in silver 4 for many seasons straight. This whole ranked system doesn't make sense to me in the first place.. I lose so many games to trolls and afk'ers and many losses like the one today I actually played very well. Who can explain why I get placed in Iron 5? For the ones curious to this game: https://matchhistory.euw.leagueoflegends.com/en/#match-details/EUW1/3861312832/25810121?tab=overview
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