Simple. Koc is ,most probably, has most experience. You should understand that today Turkish Goverment wants a Turkish car brand, desperately... I said ''desperately'' because turkish companies dont want that. And this ''New Turkish Car Brand'' is so important for Turkish goverment and Erdogan, it is not just about economy, it is about prestige. It is about votes they can earn with this way. So If lets say Koc wants to design and produce this new car, Turkish goverment will support this, because they dont have a choice.
You can give Milgem or Anadolu project to another shipyard instead of Koc. There is so many alternatives for that. But about a new Turkish car they dont have any alternatives.