I wonder how much of that has to do with the fact that ISO8601 was relatively newer back then (published 1988), and date formats were much less standardised 20+

I wonder how much of that has to do with the fact that ISO8601 was relatively newer back then (published 1988), and date formats were much less standardised 20+ years ago. For instance, RFC 1945 (HTTP 1.0) specifies three acceptable date formats. Add ISO dates, and now you're writing software that needs to handle 4 different formats. Easier to just use a heuristic parser that's capable of accepting HTTP, email, Usenet, and ISO timestamps. Fortunately, these days it's generally considered a bad idea to use anything but ISO timestamps, with the exception of a former colleague who tried arguing that DD/MM/YYYY was an acceptable format to be transmitting and storing dates in.