Comment by ksec
Performance, scalability or ultimately cost. Remember 10 - 15 years ago when Twitter was using Ruby it was a lot slower. Even without YJIT CRuby today would still be 2 - 3x faster than then. Tooling wasn't as good, Hardware were a lot more expensive at the time. So when you have news spread out about how RoR cant scale cheaply, they jump to newer and shinier things like Node.
I would guess running RoR today is 100x cheaper than 10 years before. And will continue to improve as we get ZJIT or running on top of JRuby.
When Twitter was using Rails it was also using a totally broken architecture that was way too centralised. Their replatforming also involved rearchitecting.