I have been asked that question and have asked the question myself a couple of times. There are so many different technologies that I can choose to complement my current skill set, why Ruby?
Well, I came across an interview from EngineYard blog recently - Chicks That Rip: Ines Sombra.
Ines’ answer is almost exactly word for word of my own answer to the question:
… The community, the rate of innovation and polyglot systems - this gets the geek in me excited.