1. Batista is more charismatic than Reigns.
2. Even back then, he was better on the mic and in the ring.
3. The WWE did not try and bury every over wrestler in the company with the worst-booked Royal Rumble of all time in their effort to push Batista; only one guy really got screwed up by Batista's push (Orton, because Triple H buried him several months beforehand to keep the way clear for Batista, his guy.) Meanwhile, with Reigns, they brought back Bryan and tried to bury him (again); even before that, they buried Cesaro back in April 2014 when it looked like he would get over enough to interfere with their plans to make Reigns the next top face, and then buried Ambrose when he got hot, followed by exiling Orton off TV when he caught fire, and then basically killed Dolph's momentum after Survivor Series. Note all 4 of these guys were substantially more over than Reigns was last year.
4. Not only is Lesnar not really a true heel, he's far too charismatic, far too good in the ring, and even entertaining enough on the mic for fans to not want to see the title taken off him. WWE tried their best to get Roman cheered by keeping Lesnar off TV purposefully (even though Lesnar was willing to work more dates for the fee specified in his contract), booking his title reign horribly (Cena three times in a row), and then when their efforts to get heat on Lesnar completely failed, they just kept him away from Roman during the buildup because they knew Lesnar would get cheered.
5. Let's just say 2005 Triple H is inferior to Lesnar, given he's a politically manufactured/company-made main eventer who doesn't have the same level of talent (in-ring or on the mic) many guys who haven't been booked as strongly have. He mostly had "please retire" heat in 2005, and that worked in Batista's favor. His title reign was unfathomably boring; people wanted it to end. People didn't want the belt taken off of Lesnar if it was going to Reigns.
6. Because Batista got over organically with the slow-burn face turn (and because he was more charismatic to start with, Triple H was getting more heat than Lesnar), the WWE did not have to have the likes of Heyman, Bryan, and Rollins (or the 2005 equivalents) cut promos trying to verbally blow Batista in a transparent effort to get him over like they had to do with Reigns.
7. Batista was not giving out of character interviews where he talked down on his stablemates that carried him over the previous few years, like Reigns did when he called Ambrose and Rollins "journeymen." You also didn't see Batista saying the fans "hated their lives" for not cheering him, nor did he ever say anything like "I'm just a guy getting rich." In other words, Batista has been a heel character; Roman appears to have heel qualities as an actual person, a little like CM Punk (who I liked as a pro wrestler but admit he was/is a dick), only he doesn't have enough talent/skill (in-ring or out) for people to overlook that and give him a pass the way other popular actresses/singers get if they're good enough at what they do.
8. Commentators never tried to shill Batista as blatantly as they shill Reigns on commentary now, nor did they ever try and do something like those "injury update" segments Reigns got for several weeks.
9. Even while getting pushed as a single's wrestler, Batista was not booked anywhere near as strongly as Reigns is now. He lost multiple times, to Orton and others, never set elimination records like Reigns did, and the like.
10. Batista was characterized as a powerhouse and actually had the requisite moveset associated with that billing. Reigns is billed as a powerhouse but doesn't wrestle as one at all. Harper, Ryback, Rusev, even Swagger all have more of a "powerhouse" moveset than Reigns does. Late 2010-2011 Orton even wrestled more of a powerhouse style (multiple suplex variations, Angle Slam, Powerbomb, was using the Gutwrench Neckbreaker regularly again, etc.)
Basically, it's a cocktail of the Reigns push directly screwing over people the fans like better than him (which wasn't happening during Batista's push), Batista having more talent, Reigns sucking at PR (if you want to give him the benefit of the doubt), a visible disconnect between reality and how he's presented/booked, superman booking, a lack of talent/skill/entertainment value, a champion much more likable and popular than he is, and Cena-like shilling on commentary/by other wrestlers/the script of the actual shows.