I believe Derek kinda went back on this answer a bit and ended up taking up Rails again later on, but the issues regarding PHP being more practical and flexible for him at the time were pretty important. These issues will still be relevant for many developers and even as a Rubyist myself, I'd say Rails is certainly not the right choice for everyone.
http://www.oreillynet.com/ruby/blog/2007/09/7_reasons_i_swit...