#2 Fans aren't as invested in him as previously before
The fans are beginning to turn sour on Brock as a WWE performer. His feuds all seem to go the same way: Heyman hypes up the opponent, the guys have a massive pull apart brawl, Brock beats the guy after taking some punishment with a single F5. The matches have regressed, as they used to be more intriguing and genuinely fun to watch - fans notice this and are naturally becoming less excited to watch him compete as a result.
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His one memorable match in recent memory was with AJ Styles, who is talented enough to have a good encounter with anyone on the roster. Fans are also becoming increasingly tired of seeing the part-time fighters dominating the big four main events, as not seeing titles for months on end naturally decreases its value. It's not the first time that he's done it with the world championship, either. Fans are eager to see the guys whom they've invested time into every week main event the big shows, as opposed to those who appear sporadically.
Following his massive win over the Undertaker, the shine has gradually worn off and he no longer feels like the unbeatable destroyer of his 2014/15 run. Having become overly repetitive, fans no longer want to see matches with multiple suplexes anymore - his value as a performer has reached an all-time low.