Becouse i find 100$/year for diamond membership is best 100$ spent for a player who want to improve his rating quick and with very low amount of money. There is just no better way. You receive best video lessons from many top coaches. Best tool chess mentor again, best lessons. Unlimitted tactic training and stuff like that.
Many players spent 100$ for just few private lessons for only 1 month from only 1 coach... here you get thousands of lessons (many of which are top/best) for whole year and much more.
I'm a premium member sir because I can't get enough of Chess.com. It's become my closest companion because it's always there for me. I love everything about this site and I would never settle for less of a good thing. I am learning disabled because I was struck by a car when I was a small boy. I have the most difficulty with my short term memory. This makes it very difficult for me to learn almost anything and that includes chess. Chess is my passion and my goal is to reach a 1200 rating one day. It takes me around 40 hours a week of studying chess in order to perhaps raise my rating even 50 points in a single year. Chess.com helps me to maintain a healthy and healthy lifestyle, and at a price of $100 a year I feel I'm getting a great deal. Peace!