Using an 'unfashionable' language is similar to other areas in life where one rejects mainstream or too modern things for obvious reasons:
- Be different.
- Use something which is extraordinarily suited or especially made for your task at hand (in other words: do not compromise).
- Maturity and stability is great.
Please add more. The analogies are manifold.
I really like modern re-implementations of well proven languages (Clojure <- Lisp, Factor <- Forth, ...)