Maybe ORAS didn't have the battle frontier because GF couldn't meaningfully rebalance it to accomodate the new generations' additions? Magic Guard alone basically trivializes the Pyramid and Pike, the Arena was always a pretty bizarre attraction, Palace was a shit show and notorious for being hated, most of the Brains' teams are trivialized by power creep and the phys/spec split specifically (Lucario basically solos Brandon's team). Maybe they just couldn't get it to a really adequate level of balance before release.