In other words, people don't understand the need for it. Those that do use other formats - json or toml, namely.
Writing movie scripts in YAML using unquoted strings? That's pretty contrived. Using literal style is easy when it is potentially needed (e.g. programmatic output), and any decent editor can highlight inferred types in helpful ways. I've used YAML in a variety of contexts and never been bitten by this one, and I don't think that any of his examples are still problems in YAML 1.2 (from 2009).
The Ruby security problem they reference is also absurdly misattributed. The problem there is with trusting serialized data to mark its own types, and having no limits on what types can be deserialized into. That's a depressingly common security problem in many web frameworks, and YAML as an interchange format isn't a unique source of vulnerability. Any data format is dangerous on the web if you trust it to create arbitrary types.
irb(main):002:0> YAML.load("on: yes")
=> {true=>true}
https://github.com/crdoconnor/strictyaml/blob/master/FAQ.rst...
This was my attempt to cut out 90% of the crap, leaving the nice, readable core (which maps on to JSON pretty neatly):
https://github.com/crdoconnor/strictyaml