Brock's always been a mercenary who'll work if he's paid and do what he's asked to do. The guy lost his first match back CLEAN, lost CLEAN to a washed-up Triple H, and didn't win a single match clean in his return tour until he faced Taker, as crazy as that sounds. He even was booked to be scared of guys he bulldozed in his first run (remember the cringeworthy Big Show stuff?)
Basically, WWE could have had him work all the big-4 PPVs (and maybe even more than that) every year he's been back and just had him do pre-taped vignettes and interviews when need be, but they're too cheap to do it and wanted him to get booed so badly they basically ordered him to stay at home so people would turn on him for not being there.
Thing is, people WANTED to cheer Brock when he came back. I still remember that huge pop he got when he returned in 2012. And people were probably nodding in agreement when he said that people were "tired of John Cena's bull!@#$" in that pre-taped interview segment.
Too bad WWE ruined Brock like they ruin damn near every good thing they have these days.
We could have seen Brock/Orton, Brock/Rock II, Brock/Sheamus, Brock/Cesaro, Brock/Swagger, Brock/Del Rio, Brock/Ryback, Brock/Batista, Brock/Ziggler, Brock/Bryan...the possibilities seemed endless when he came back. And much better than 4 matches with Cena, 3 with Triple H, 1 with Taker, 1 with Big Show, and the 1 with Reigns where he's going to lose.
Also, a huge potential missed opportunity for one of the biggest matches ever at EC 2014; if the WWE hadn't screwed with Punk and made him want to leave, try to imagine a hypothetical main event of Brock/Batista/Orton/Cena/Punk/Bryan. That could have happened.