Well, the idealistic excuse would be that the Player kicked all their asses into the next realm, but HE COULD NEVER DESTROY THEIR RELIGIONS!
Option B) The Gods came to some sort of a Gentleman's agreement, and decided that rather than have the entire landscape end up like the Wilderness and not have anything left to fight over, they'd have champions of their respective religions go out and conquer Gielinor whilst they battled it out on a separate realm. Of course, each God either were fully confident that they would win or intended to disobey the agreement should they lose, they all went to a Private realm, and the battle drew to a stalemate: No God allowing any other God to Leave, yet their power levels too in-balance for any one to beat any other.
Meanwhile, in Gielinor, each Gods' Respective factions have gotten started
Option C) The Death of Guthix and his lasting effect somehow meant that the Gods were limited in Power when they came to Gielinor after the end of the Edicts. This posed a massive problem, and meant that it'd be more advantageous for an individual to possess their support and then have them step in to command the empire after it's fully established, rather than themselves set up the empire entirely.
Option D) An Elder God (Williamicus) used his Elder-god powers to suck out all the gods, and essentially became the New Edicts: GODS CANNOT INTERFERE!!!!
Option E) We could just let it be a Mystery.
EDiT: Also, if you were a Traditionalist, then how come Guthix has died in this Universe?
I wouldn't mind some of Slithering Sliske's Super Slimy Supporters running around in the Frozen North.