Why is diamond so hard to get out of

https://preview.redd.it/o8tidawn1sne1.png?width=236&format=png&auto=webp&s=2635b4e0824f7f121a2bd81a3e8eb28ba72eadf2

Every year i get to d3 - often div 3-4 (normally stay there) and after no progress for weeks and then deranking leads to me just end up quiting. Then i get back into the game and the same thing happens again. Why is diamond so inconsistent? My mechs arent great but i play solid fundementals. Im just hardstuck