The thing is that Breloom is not only a sweeper. Breloom can put one Pokemon out of the game with Spore, except from a few select cases, has strong priority, and is very versatile as he can run sets that all require different counter and/or checks. Terrakion has none of those. Sure it can sweep better, but Breloom can do things other than sweeping, which is where its value lies. If only the SD set existed i wouldn't propose Breloom for S rank, but with all the different and effective sets he has, he is quite a force and is one of the best Pokemon i have used the past months.
Part of the OU QC team, message
me for a check!