Level 30 gets silver 4 after winning all games

So about 4 years ago my account got banned for a week so I made a new one to keep playing. This account was completely forgotten over the years until i tried to change my summoner name to it and remembered it. I log in, forget i only have about 40 champions but say fuck it and play ranked on it anyways. I win all 8 of my provisionals and get Silver 4. If I remember I was getting placed with high golds and plats in normals before I stopped playing. Why would my mmr decay so fucking drastically for this bullshit to happen.
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