I don't have a link to the post on me right now, but I do recall the Developers saying at one point that a Cthulhu boss would never be added.
Aside from that, in my opinion, it felt like Terraria has never needed a Cthulhu boss. Just because we fight the body parts of Cthulhu doesn't mean that Cthulhu as a whole needs to be fought. Aside from that, as mentioned by UltiDaniel, the Moon Lord is Cthulhu-like. More than that, it feels like Moon Lord effectively is Cthulhu, that was my 1st impression of the Moon Lord when I fought him. It is as close to a true Cthulhu fight that I think we will ever get in Terraria, and the eyeballs in it's forehead and palms that it has referred to as the "True Eye of Cthulhu" seem to further give the implication that the Moon Lord basically is the Terraria version of Cthulhu. I think it's enough also, as the Moon Lord is a challenging and satisfying final boss, and it wouldn't make sense in my opinion to have more bosses after the Moon Lord.
I'm aware that Cenx has stated before that the Moon Lord is Cthulhu's brother. This is just my own perspective on the "aura" I get out of what the Moon Lord is.