Okay,

Arkham Horror is basically Call of Cthulhu but why does FFG bring out a new edition CoC game which doesn't use d20?
Other than not having the rights...

I don't quite understand what you mean. Arkham Horror isn't an RPG game, but a Call of Cthulhu Boardgame. I'm sure FFG has all the rights (licensed from Chaosium, etc.) to create this game.

I'm confused, too, actually. Chaosium made Call of Cthulhu, and also worked with Wizards and made a d20 version. It ran out of its license, but the books are floating around somewhere (internets or book stores). Don't look on Wizards' site, or Chaosium's though. Since it involved a joint project, when the deal was off I'm pretty sure neither could sell/advertise it anymore...

...and if your going to play Call of Cthulhu may I recommend The Complete Masks of Nyarlathotep and Delta Green as two of the best role-playing supplements ever produced.

Santiago said:

Well, the Mythos is public domain (the parts HPL wrote anyway) and d20 (3rd ed) is still available under the Open Gaming License. As far as I know there's no reason for the OGL on 3e to become invalid just because Wizards have made a 4e subject to its own rules. In fact, I'm pretty sure that's the entire basis behind Pathfinder.

So publishing CoC using d20 would leave a large chunk of the game open to free use by whomever. It would also beg the question of why buy the new edition when people already own CoC d20. Making a 4e CoC would be a pain, from what I understand of the 4e version of OGL, so they just make their own engine which they can put their own stamp on, like it used to be in the old days.