TLDR: it's just usually not worth enforcing this on lint level.
Generally, variables and properties should be camelCase or PascalCase. But elision variables like _ should be allowed as well. Numbers are sometimes useful as well. And don't forget those good old ALL_CAPS_CONSTANTS! Now you have a RegEx like this:
^(?:__?|[a-z][0-9]*|([A-Z]?[a-z]+)+[0-9]*|[A-Z]+(_[A-Z]+)+)$While that's a rather practical constraint, there are often some exceptions that just don't fit. Additionally, most projects do that anyway, and consequently don't profit from this rule.
“There are only two hard things in Computer Science: cache invalidation and naming things.” — Phil Karlton
Also, with that👆 quote from the rule page in mind, the main issue with naming is usually not a technical issue, but one in regard to meaning: the matter of whether to write someVariable or some_variable is usually ease to resolve, less so whether to write buttonBig or bigButton (word order), even less which of buttonBig, buttonPrimary, or callToAction (descriptiveness). The latter, interesting ones are impractical to enforce with just a regular expression.