... Why do they need more then one codex? I mean that spend more then half of their time working on new stuff for space marines.
Spacemarines sell and can be produced with very little effort on the part of GW's design team
To make a new codex all they have to do tweak a few points values, make up a Space Marine Character or 2 and if they are feeling extra-specially energetic make up a new vehicle or troop type or some minor variant on an existing one. Fill out the rest of the book with stories of Space Marines in (supposedly) hopeless battles where they win against the odds (supposedly).
Whole new Space Marine armies can be sculpted by just making new symbols to go on the should-plates and asking the painters to paint them a different colour.
Space Marine Characters are also very easy for them to sculpt, pose some existing Space Marine armour semi-dramatically and sculpt a few extra details onto it and add a bare head and Sword, Power Fist and/or Combi-weapon
Finally make a big fuss about it being "the bestest army evva!" and all the kiddies will come buy it
This is why since 3nd edition, Space Marines have had a separate Codex for each of the major variants on the army each edition and Orks have 2 Codexs to cover their race in that time that barely mention the possible variants in Ork Kulture and Armies (Sorry I rant, but Orks were my favourite 40k army and got totally ruined in 3rd edition and had all their best equipment and weapons taken away making them unplayable)
Of course other races in 40k are equally neglected, some barely get an update and 1 doesn't even exist any-more! (although it may end up being desperately re-released toward the end of this year because another company is doing a similar army for their own more affordable Sci-Fi game)
~The ravings of a single mad Goblin is bad enough, but such a power-hungry, malice-filled creature as Mortis can never hope to be understood~