To be a bit more exact, from 3rd edition onwards we've had:
3rd Edition:
Space Marines, Dark Eldar, Blood Angels, Chaos Marines, Dark Angels, Orks, Eldar, Guard, Tyranids, Space Wolves, Necrons, Tau.
3.5 Edition:
Chaos Marines, Armageddon (intro the Templars + Salamanders, Steel Legion & Speed Freaks), Catachans, Kraftworlds, Dark Eldar re-print (added vehicle upgrades + pts re-ajustments), Dark Angels re-print (pts ajustments & a couple new options like generic Termie Captain), Daemonhunters, Witch Hunters, Eye of Terror (early draft rules for new age Guard + all-Guardian Ulthwe + 13th Company + Lost and the Damned)
Pre-4th edition:
Imperial Guard (w/Doctrine system)
4.0 Edition:
Space Marines (w/Chapter Traits system), Black Templars, Tau, Tyranids.
4.5 Edition:
***Note, major design shift here with the rules being hugely dumbed down and options being thrown out the window! This included, Dark Angels, Chaos Marines,
WD Blood Angels, Eldar.
Pre-5th Edition:
Orks, Chaos Daemons
5th Edition:
Space Marines, Space Wolves, Blood Angels, Tyranids, Imperial Guard, Dark Eldar, Grey Knights.
Pre-6th Edition:
Necrons
6th Edition:
Chaos Marines, Dark Angels, Tau Empire, Chaos Daemons, Eldar, Space Marines, Guard, Tyranids.
Black Legions, Crimson Slaughter, Farsight Enclaves, Iyanden, Clan Rauken, Sentinels of Terra supplements.
7th Edition (introductions of unique
FOC's/Detachments):
Orks, Space Wolves, Dark Eldar, Blood Angels, Imperial Knights, Grey Knights, Clowns, Admech + Skittari.
Waaaagh Ghazgkull, Haemonculus Covens, Champions of Fenris, Shield of Baal supplements.
7.1 Edition (introduction of the Decurion/Formation armies):
Necrons, Eldar, Khorne Daemonkin, Imperial Knights, Space Marines, Dark Angels.
Whew! I'm pretty sure that's everything excluding the likes of the "codex"
LotD and other similarly limited
BS dlc
GW tried through 6th edition...
As it stands right now, the most hard done-by army is easily the non-existent Sisters, who like the Inquisition, are now restricted to e-codex only
BS and thus, have been left out due to not being available in standard printed format.
Outside of that special case, the most hard done by armies nowadays are easily Chaos in general & Guard.
Tyranids at least got a bunch of new models with more up-to-date rules, plus the added formations in the Shield of Baal books. Guard & Chaos though are trash level
atm, as both require huge crutches to lean on...
IoM ally shenanigans for Guard, and Summoning/Flying Circus for Chaos in general.
I don't think this is so much a problem of there being 'too many factions', but rather that
GW gets too dysfunctional & highly impatient in their codex design philosophies... Typically, they decide after a half dozen or so books to radically alter their design premises, and we either end up with a huge leap in power creep, or else the very glaring catastrophe of mid 4th edition whereby 4 books were pretty much shredded of their very souls and left with a hollow shell of an army list.
With their current ramped-up release pace,
GW simply needs to stick to this current trend of 'Decurion organisation' for the books that don't yet have them!
Of course, this is
GW, hence they'll likely decide after the next two books, (my bet is on Tau & Guard), to radically alter the force organisation again, and we'll get Tyranids & Chaos being royally screwed over, yet again.