It was rumored that Regal was fired from WCW for making Goldberg look like a fool in a televised match.
The writing was on the wall for Regal in WCW once Hogan came in. Prior to Hogan's arrival Regal was getting a near monster push as a heel, he had been aligned with Stunning Steve Austin, feuded with Sting, and had an excellent Best Of 5 (might have been 7, I cant remember) series of "Strict Rules Wrestling" Matches with Ric Flair. He was dominating mid card feuds and acquitting himself well against top stars.
Once Hogan arrived there wasn't as much attention paid to the mid card, at least not initially (that changed in the early Monday Nitro era). Regal didn't have the size to be considered a major threat to Hogan and with The Hulkster on board Flair was turned heel & The Four Horsemen revitalized, pushing him further down the proverbial ladder. Regal was never a player in the Flair/Horsemen-Sting-Luger-Savage-Hogan main event scene and when WCW did start paying more attention to the undercard it was mostly cultivating the cruiserweights, he was left behind.
By the time the match with Goldberg happened he was already on life support and expendable which was a shame because he snobbish British aristocrat heel character was catching on pre Hogan and working well with Sting & Flair when he was paired with "the big guys".
In WWE Regal never had the "look" or "size" to garner a huge push but he managed to stay relevant in the mid card for several years before moving to more behind the scenes oriented work, where he finds himself today. He must be in good standing with the company because they keep him around which says something for his talent level and contributions.
I think if Hogan hadn't gone to WCW - or at least of Eric Bischoff would have been a much better manager of talent - Regal would have done quite well for himself and found himself in the that "top tier" of villains that may not main event but do quite well for themselves, like Rick Rude or Jake Roberts. His size & style were never going to get him a huge push in WWE (remember his lumberjack character ??) but he did overcome that and manage to be a significant of not superstar player for several years before moving behind the curtain. Its not a bad run by any means but it with a little luck it could have been much better.