Because Rails was a sufficiently easier onramp for simple database-backed web programming than Java and sufficiently cleaner than PHP.
I'll let you ponder that for a second, until you're no longer composing angry responses to this article. When you've convinced yourself that that's indeed correct, the next paragraph is one blank line away.
I've oversimplified, of course. Luke Kanies (creator of Puppet) told me directly that he wrote Puppet in Ruby because he wanted to learn Ruby. (I won't claim that Adam Jacob told me anything similar, but I can believe it.)
If Rails was Ruby's killer application in 2005 and 2006, Rails brought new attention to Ruby, and that new attention brought Ruby to new places. The Internet will certainly debate whether Rails is Ruby's primary driver right now, and the Internet incessantly debates whether Ruby or Python or Perl or shell or whatever is better or more popular or more fashionable for non-Rails tasks such as administration or automation, but in 2011 your eyebrows don't automatically raise themselves when you run across an automation tool written in Ruby as they would in 2005 or 2004 or 2001.
The sudden popularity of Rails brought a second wave of popularity for Ruby for non-Rails.
I suspect you don't see the same popularity for command-line PHP because no one adopts PHP because it's better than something else. People adopt PHP because it's easier than everything else.
(I also don't know how to characterize Node.js, except that Netscape tried it back in the '90s and it didn't work, and it reminds me of the AOLServer dilemma, except that Node.js has a little more shiny for some reason. Maybe a JIT and something resembling a working module system will do that for you. Then again, Perl's second wave came because system administrators already had a pretty decent language for system administration, and isn't it easier to write web programs in this than in shell or C? (I own a book about web programming with the Korn shell. I am not making this up.))
Conventional thinking during Perl 6's beginning claimed that Perl 6 would be the natural evolution of Perl 5, just as Perl 5 was the natural evolution of Perl 4. The RFC process demonstrates this—we want a better object system! Better threading! Improvements here! Improvements there! More consistency! Fewer edge cases!
Even the early talk of Parrot and Ponie bore this out. At some point, Perl 5.12 was to run in Parrot, and the two languages would merge together.
Some people are happy that didn't happen. I admit I don't understand their view, but I'm honest enough to admit that they may have a point. I want a language with a great object system, with malleable control flow, with macros, with native grammars, with optional typing, with an optimizing compiler, with better garbage collection, with a great native interface, with serializable bytecode, with proper language introspection, and with finally and ultimately function signatures, and, yes, access to the CPAN.
I don't have 14 years of legacy code to worry about, in part because I have App::perlbrew, but in part because I run my code on the latest stable releases of Perl as they come out.
Maybe Perl will gradually become Perl 6 over the next decade or so and solve that problem, but I have to write and deploy code on October 14, 2011, so that doesn't really help right now.
It should be pretty obvious that if Perl 6 ever comes out and ever gets any adoption, it won't do it by being a better Perl. At least, at the time of this writing, there's no evidence to suggest such a thing ever happening. (Don't confuse the fervor of a few true believers with evidence of anything but the fervor of true belief.)
In other words, if you're looking for the pragmatism of Perl with some of the language flaws fixed but with access to the CPAN, Rakudo isn't it and won't be it for as long as anyone can foresee.
There is another path.
I first used Ruby in late 2000 or early 2001. Dave Thomas told me about this Rails thing in August 2004. Rails took off in January 2005. While I don't believe that the web will eat all software ("The script on this page is taking a long time to recompile your IDE. Would you like to stop it or continue waiting?), Rails did offer those poor plebes in the J2EE and PHP worlds something far better than what they had.
The problem is that Perl 6 has no reason to exist. There's no singular problem that it solves. It has no users and no use cases, and the only user feedback it gets from people who aren't interested in it because they believe it's fun to write a compiler is "It's slow and buggy and you can't do anything with it."
It's difficult to bring a product to the market without customers and it's difficult to find customers if you don't know what they want or need.
Thing is, you can't easily predict what that will be or when and especially why. You can't optimize for that based on user feedback. Even so, I trust Larry's instincts and good taste. I believe him when he says he believes it's possible to avoid Worse is Better and, for once, get a Better is Better cycle.
Maybe the best way to explain what Perl 6 is, as it stands today, is as a revolution in search of a cause, but that's not exactly a roadmap to the success of relevance.