it's not that fire is that bad.. it's just that you can only max 2 force mastery.
so if you gotta pick between fire,light or cold then fire has less things you need or could use then the other 2 options.
Maybe this post helps too. It's about why most ppl choose cold and light and not another option:
(from a similar question in another thread about nuker builds on 90 cap)
flexo wrote:
okay the way i see it:
almost everyone says bicheon maxed for KD, iron wall etc. now as for the final 210 mastery's you can pick 2 tree's and have 30 mastery spots that you can put where-ever..now about those 2 tree's
light has the mag dmg increase and the strongest nukes at the 90 cap.
cold has the phy def and mana shield that makes a nuker able to tank.
fire has the strongest imbue and highest mag def (and also highest phy dmg)
so most ppl focus on strong nukes and tanking ppl (with the new snow shield serie) while they nuke. So they go light/cold.. ligh imbue ain't really that much weaker then the fire imbue so the sacrefice of leaving fire out really only lies in the decrease of mag def and phy dmg.
or looked at a bit diffrent: pure INT's have low phy def and do high dmg (mostly mag). So like any build you wanna cover your weakness and increase your strenght. So more phy def with cold and more mag dmg with light (pierce force serie)
Ofcourse other options are possible depending on how you fight and what kind of fights you are gonna be in. But cold/light generally makes the most sense.
ppl are planning for the 90 cap because the rumor goes that the 90 cap will come soon.. as in within 6 months.. personally i don't believe it.. but then again.. that is based on the idea that JM can't pull it off imo ^^