Since returning to WWE in 2012 after a successful stint in the UFC, Brock Lesnar has been a part-timer, competing only at major PPVs and appearing in non-match capacities on Raw and SmackDown. During this tenure, one in which he's adopted the near-unbeatable persona of the Beast, there haven't really been any opponents or storylines that truly measured up or offered a believable challenge.
He beat John Cena mercilessly when he came back, broke The Undertaker's WrestleMania streak and even brought an end to the legendary Goldberg. However, apart from these runs, there's nothing truly memorable, and given the recent feuds Lesnar's been in, it's clear WWE has no idea what the hell to do with the current WWE Champion.
Lesnar didn't have any real contenders for quite some time, not until he and Goldberg fought in 2017 and Lesnar took the WWE Universal Championship from the veteran. It wasn't until this year, though, that Seth Rollins officially became the "Beast Slayer" of Raw and won the title from Lesnar that there was something fresh for Lesnar and his hype man/manager Paul Heyman to contend with. However, in the wake of this dethroning, Lesnar went to SmackDown and squashed Kofi Kingston for the WWE Championship and defended it in a No DQ match at Survivor Series where he battered Rey Mysterio and his son Dominick in an easy title defense. To make things worse, Lesnar didn't even appear on the following Raw, which once again affirms he's a part-timer without a plan.
Many fans felt this was one of the reasons Vince McMahon's creative team took a major championship off Lesnar, as people wanted to see it defended. Lesnar wasn't doing that, which is why fans clamored for him to drop it to Rollins. Without a title, though, it was up in the air as to what Lesnar would do next, with WWE deciding his former UFC rival Cain Velasquez would be the perfect opponent. This fizzled out quickly as Lesnar submitted him at Crown Jewel in October, effectively making Velasquez a joke rather than a powerhouse who was initially touted as a destroyer.
This means we're back to square one, as Lesnar has moved back to Raw and is now without anyone to challenge him. Finn Balor's gone back to NXT and their last feud didn't register on the radar, Mysterio just didn't feel like a credible rivalry so there's no interest in a rematch and it'd be overkill to see Rollins starting a third major beef with Lesnar in two years. Seeing as Rollins just turned heel, as well, it's unlikely he'll be going after Lesnar, which means there's a vacant spot. Other than Drew McIntyre, though, there's no one legitimate to challenge the Beast, reiterating how counterintuitive it was to give Lesnar a title again.
Fans don't want to see him rehashing opponents, so keeping Lesnar without a championship would have allowed him to cameo as he sees fit, beating down random people with a true purpose. Furthermore, taking Velasquez off the table so early with a clean loss doesn't make him a credible threat, so when he returns, all he'll be is the man Lesnar wiped out on debut. It'd be hard to even get fans riled up to see that rematch so again, by squashing Velasquez, WWE's running out of Superstars who could derail Lesnar or give him a run for his money.
Velasquez injuring Lesnar would have actually been a win-win for all -- Lesnar wouldn't have lost clean but would have had the belt taken off him while he lay on the shelf. This would have made Velasquez a monster and created a berserker return/revenge arc for Lesnar while giving the Beast the time off he's always so keen to take. Most of all, the title would have remained vacant, allowing Raw's roster -- which includes new faces like Aleister Black -- to get a run at the championship. However, instead of opening up the title chase, WWE creative simply dug the Lesnar hole a bit deeper, killed off someone we thought would be a titan in Velasquez and relegated the WWE Championship to something that only makes a rare appearance. Most of all, there's no mileage for Lesnar, even when he does show up, because when it comes to the rest of roster, he's been there and done that before, having brutalized them all with consummate ease.
Simply put, there's no one else, and this leaves the Beast as the king of the hill at a time when fans want to see someone stepping up to the plate so his character doesn't become stale.