There are a few things.. First of all Frost will do more damage on pure ST if the fight is long enough and you don't have to move. But, there are no such bosses currently in EP. Either fights are way shorter and benefit from 2 minute cd that fire has and from pre casting Azshara trinket at the start. Or have mechanics that benefit fire hugely.
Sivara is too short and you get owned as No-IL if you get the Frozen debuff.
Behemoth really ain't that bad for Frost, but again it is quite short of you 2 platform it.
Radiance is decent for Frost but again, it serves Fire quite well with ignite automaticly cleaving the small adds and if I remember right you get combustion back on the add phase. Also you should 2 phase it by now so the fight isn't that long anymore. But again with positioning and abusing the fact that you make the adds spawn in certain location can make Frost even or better on Radiance.
Ashvane should be 2 phased by almost everyone now. So the fight will be short and as we are saving CDs for the shield breaks, Fire will have a massive advantage, not to mention Fire can use Firestarter for the first 10%, which is just massive when everyone is saving their CD for the shield break.
Orgozoa, Fire will spread Ignite the whole fight to all mobs, so naturally it will be better. Although playing Orb build on this boss can make Frost better than average, but you have to totally redo all your gear and traits.
Court, Fire again will spread Ignites, but playing Frost well here will bring you fairly close to Fire.
Za'qul, Ignite again and Fire has a massive burst compared to Frost if you are chosen to go into the other realm. That said on heroic at least(I quit before getting to Mythic) if you whore for logs you can just kill the small adds that have the damage taken debuff and you get massive logs. Although at least in my guild when people found out after few weeks of 98-99% the adds started dying instantly because they didn't want me to abuse it.
Azshara, Fire is just leaps and bounds better on this. You getting the buff is nearly useless as Frost, while as Fire you just go balls to the wall. Fire cooldowns also fit the flow of the fight far better.
Again I want to stress that if there was a pure patchwerk boss that lasted longer than 3 minutes still on farm, Frost would likely wipe the floor with Fire. But EP just simply isn't a raid where you can stand a nuke a lot. There are a lot of mechanics that need movement, immunity phases, saving CDs, adds that you cannot cleave as Frost. Everything that makes Frost not good compared to Fire.
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