#1. His appeal isn't the same anymore

The reality is that while Brock Lesnar maybe one of the last "larger than life" superstars left in WWE, his appeal isn't the same as it used to be. For a few years after his return, things were good and he was a solid box office star.
The part-time status made sense as well. Now, the issue is that Brock Lesnar has been the same for the last few years, and so have his matches. Sure, we might get a couple of exceptions like his Survivor Series clashes with AJ Styles and Daniel Bryan, but overall, it's all the same formula.
Did you know a top WWE Superstar just wrestled outside the company? More details HERE
Every Lesnar appearance involves him jumping around while Paul Heyman cuts the same promo over and over again. His matches? Mostly just 5-7 minutes of suplexes and F5s and nothing else.
It's easy to see through it, and Brock Lesnar simply doesn't feel like the same special attraction that he once was.