Because "Ruby finally is boring."
Ruby is a very nice technology to build web apps, however the hype stopped, and most libs and frameworks in the ecosystem are now stable and battle-tested. All the cool kids who create new frameworks every month and try 'n see what can be done with a platform switched away to NodeJS. They already found all the good and bad stuff about Ruby, and its benefits and shortcomings are well known and understood. It's the best time to be productive and create stable products, which will be supported for a long time.