I think the dominant reason why ruby lost out was because python won the educational market; there was something seductive about forcing children to use good indentation. The final blow came when the difficult 1.8.5 -> 2.0 transition made the act of installing ruby a challenge, of course this happened to python not too long after with the 2.7 -> 3 transition.
Frankly, Ruby lost due to the lack of libraries compared to Python. The latter had a few years of head start.
Anecdotally, I've heard the syntax may have been the cause. In those early days, Ruby was too much like Perl. If you didn't like Perl's syntax and philosophy, Python was a good alternative. If you were the type of person who grokked Perl, then why go to Ruby when you have all of CPAN at your disposal?
Precisely. Ruby was such a hit with rails they never bothered creating an ecosystem outside of the web.
JS doesn't have this problem because it has the terribly unfair advantage of a monopoly of the most popular platform in the world.
But Ruby did not, the server was open too all. And so when competition arrived on the backend, people chose a language that you could use for the web and something else.
