Systems that already exist need to be maintained. Why do people still write COBOL? Because it's cheaper to maintain an already in place system than to rip it out and replace it with the newest fad.
By the way, Rails is the craze from maybe 10-6 years ago. consider e.g. [0] But it is still far, far from dead. Time flies, but not as fast as you suggest.
[0] https://www.google.ca/trends/explore?date=all&q=ruby%20on%20...