User blog comment:MEEEE123/Cthulhu Banns/@comment-27220028-20160125084233/@comment-25543325-20160125151949

No worries, it takes a bit of time for community to notice new threads/blogs - They'll get to here eventually.

Answering your question: Even though I wasn't the one who created this rule I can definetely understand reasoning behind it. Let me explain: ...Cthulhu was never anything else than just final boss concept - Both to Devs and normal players alike.
 * First and most important, the way you're thinking of Cthulhu in terms of game progression. Somehow, not even sure how, all new users who came to the same, exact idea always wants our favorite tentacle-ous monster to be at the very end of player's progression. Even now after the Moon Lord came out some players still wish a new "Final" boss just to fit their expectations. Really, Terraria is done. Game's finished. No need for that. We do not need any more end game bosses.
 * The War Mode is an exception here as it's supposed to take place after the Moon Lord, therefore requiring yet another boss to seal up. However, Brick Creeper already got it covered up in his The End event.
 * Next comes the idea itself - Come on guys! Why won't you come up with something more original? Funny thing is - It was never officially confirmed whether the Eye of Cthulhu's name was in related to Lovecraft's stories - They could just borrow the name.
 * Keep in mind, however, that I did not read any of Lovecraft's books, heck, I don't even know much 'bout the author himself. If you did, you might be able to point out some obvious references I missed. In such case, go right ahead.
 * I believe Moon Lord being Cthulhu's brother was just a joke. Same goes to "leaked" concept art of Cthulhu himself that Crowno posted.
 * Lastly, Cthulhu bosses are annoying. I mean: I bet you can find dozens of such just by quick google search. There are so many of them, believe me, I browsed through most on Terraria Forums; I found nothing unique, just classic, repetitive concept of super powerful end game boss...

I'm pretty sure rest of the staff will agree with me.