just wondering why the other brands besides holden and ford have not been added to the game yet

Not likely to be added now!

Forza Horizon 2 just came out and will have support. There will still be some DLC for FM5 (or so they’ve hinted at) but I doubt any new V8 supercars will be in it.

Best bet is to post it in the Wishlist thread and hope for FM6.