The Unicode people don’t sit around and invent characters – nor do they systematically determine the complete list of characters that should be in the standard.

Instead, they consider proposals from people who want their already-in-use character sets to included in the standard. So if someone proposes SINGING OCTOPUS, DANCING OCTOPUS, and SINGING SQUID, but not DANCING SQUID, well, DANCING SQUID won’t be in Unicode. There are lots of gaps like this.