I never bought into it, so I can't say for certain, but I think a big part of it was that RoR had some serious security holes. Well, so does PHP, but it remains popular.
Beyond that, while Ruby seems OK with Puppet, the Ruby gem stuff for Ansible is a continual nightmare for me. I don't know if RoR had that problem as well, but if it did, I could see where the fantasy of getting rupee-a-day junior programmers to crank out websites by the score would wilt pretty rapidly.
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I don’t have my finger on the pulse of most major trends, either up or down — too busy working on Rails to step back and see what the world thinks, I guess :-) But in general I think popularity always has some wobble. It could well be an ongoing process of people exploring what are and aren’t Rails’s sweet spots — that can account, I find, for a lot of the volatility in any technology’s popularity.
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I don't think popularity has waned much, if at all. For awhile there was a lot of buzz about Ruby and how it was going to topple the giants - Java, Python, C#. But that never happened, and when people look back on, say, 2010, and compare it to now, they don't see the same level of excitement or hopefulness. That's understandable, but doesn't necessarily reflect a decrease in usage or, certainly, utility.
I view the history a little differently. I love Ruby (obviously), and I think the language maintaining its status as an underdog gives it distinct advantages. It doesn't have to support every release ever made (Java) or lock itself into only one right way of doing things (Python). It's free to experiment, take suggestions and contributions from an active community, and cultivate it's already strong following. In programming, as in life, the most popular things are not always the best!
some Ruby developers look into Elixir/Phoenix ecosystem after a certain point. even though its a huge different stack from object-oriented to functional paradigm and highly concurrent design but syntax feel the almost rubyish style and phoenix framework do a great job like Rails by making things feels awesome.
Erlang VM bring the real value here, its Distributed nature, Fault tolerance and high availability do the real magic.