Ruby is very flexible. Is that the issue?

I suspect not, because Scala is super flexible and probably more difficult to parse, but has a better organized parser.

The language is just quite large and quite complex to parse - context-sensitive and requires interaction back and forth with the lexer as it parses. TreeSitter seems to have a much simpler Ruby parser - but I believe it ignores the context-sensitive parts of Ruby.

Ruby's syntactic flexibility is definitely a big factor in it.