To fully understand the reasons behind Rails' design choices, it is beneficial to very briefly learn about its history.
Rails was released to the world back in July 2004. The framework was extracted from a Web-based management and collaboration application project called Basecamp. Rails was created by David Heinemeier Hansson (often referred to simply as "DHH"), a Danish programmer and partner with 37signals, the firm that produces Basecamp and other similar Web applications. This brief piece of information offers us some important preemptive insight.
Rails was not designed by a committee. It was extracted from a real-world application. Even after Rails' incredible success, David has insisted that there won't be a Rails, Inc. because he firmly believes in the importance of working on real applications and only then applying the most useful lessons learned (and possible missing features) back into the framework.
This approach guarantees that Rails doesn't end up becoming a bloated framework that includes all sorts of features, to satisfy the requirements of any possible company or scenario out there. Rails is intentionally general enough to be used for a wide range of applications, but its focus has always been the needs of 37signals and other companies/developers who take up similar principles. That's where Rails really shines. To paraphrase what David said during a keynote at Startup School (http://startupschool.org ...