@Misadventure <333
More seriously, the long answer is that Stardust and I both loved playing Geist, but found that their power levels escalate swiftly enough in comparison to other spheres that it eventually makes game-wide plots difficult to balance: when one mid-level PC can just literally drop a building on any antagonist or facility (or PC), most other spheres' participation become redundant.
Or, you end up having to stat NPC villains so high that other spheres can't really touch them.
After seeing how disruptive that became on several games (our founding staff have all staffed on nWoD games before), we decided, at least for now, to hold off on including them. The other option would have been to exclude them from many multisphere plots, and we didn't really think that was fair to Geist players.
It sucks, because they're really cool and have a lot of good storytelling and plot stuff to mine.
Same reason we also don't have Mage, though.
There's also the issue of a new game offering so many splats that it splits the playerbase too much. Right now we're at about, uhhh, 82 currently-approved characters, and already are dividing them between Vamp, Changeling, Ferals, immortals, and mortal/+ (and very slowly Promethean). Vampire right now is disproportionately strong and active, so adding new major splats when we're focusing on getting 'Ling and Feral (especially) the activity and attention they deserve would just be counterproductive.
I want F&L to be a game that offers everything to everyone, but I don't think that's responsible to try when our game is just a few months old. I think that the trend of single-sphere games lately is an acknowledgement of how too much variety can stifle roleplay.
So at present we're just offering the spheres that are mostly in balance with each other, in terms of power.