Money-wise, he made enough money and saved it well enough that he felt it was much better leaving the WWE at the time than sticking around to make even more money.
There was reports at the time how he was upset over the direction of his character but that was because WWE didn't just want to make him with another 100+ win streak and his matches required more effort than he was used to in WCW. Once you take away his streak and require him to wrestle more, his character didn't seem as amazing as it was in WCW.
The bigger problem for the WWE at the time was Brock leaving since they invested a lot into him as a future star and he ends up leaving the same time as Goldberg. Also bad was putting Goldberg on RAW instead of Smackdown since the main event was already crowded and stale during the time he was there unlike Heyman's era on Smackdown.
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