It's just so blatantly unnecessary to support any file encoding other than UTF-8, supporting "extensible data types" which sometimes end up being attack vectors into a language runtime's serialization mechanism, autodetecting the types of values... the list goes on and on. Aside from the ergonomic issues of reading/writing YAML files, it's also absurdly complex to support all of YAML's features... which are used in <1% of YAML files.
A well-designed replacement for certain uses might be Dhall, but I'm not holding my breath for that to gain any widespread acceptance.
 Present tense. Things looked massively different at the time, so it's pretty unfair to second-guess the designers of YAML.