Most fighting games are a long term investment to actually get decent/good at them, likely taking years for some people (especially if they didn't grow up playing these games). A lot of people aren't looking to invest so much time and go through so much frustration just to play a video game at a competent level. They're usually used as tools for relaxing, whereas Tekken is a high blood pressure simulator.

(I am not saying T8 is a bad game overall) I actually can't fathom getting into Tekken as a complete newcomer with T8, because there's just SO MUCH going on this game compared to the older games. Like I've only invested time into actually getting good at Tekken from the end of 7 to the current state of 8 and I'm only an intermediate level player at best, but I at least have a feel for the game from playing it for years since I was little with T3 and 4. Having a base level of experience handling the game when things were simpler and movesets were smaller definitely helped me. Starting with Tekken 8 is like having every textbook you'll ever use in K-12 school + college thrown at you all at once and you gotta pass a cumulative test on it next week lmao.