First of all, Ruby has a very ambitious goal for version 3.0, which most likely involves some kind of thread parallelisation.

Second, I would not say Ruby is not a language for the multi-core world. There are tens of thousands Ruby production environments over the world that make excellent use of multi-core machines. They run Ruby on problems that are embarassingly parallel such as web applications using process managing application servers such as Phusion Passenger or Unicorn. This is the way many modern web application platforms parallelize, including Node.JS and Python.

Third lack of paralellization is not a language feature. Both JRuby and Rubininus support threads without a GIL. Projects like Celluloid and Concurrent-Ruby make use of this.