Same reason as you'd learn any language: job market (some areas have more openings for one language (for instance, in Houston, Ruby is slightly ahead of Node, and C# blows both of them away) or another or you need to work with projects written in that language (in the Ruby example, say you want to write a module for Metasploit or hack on Gitlab), etc. It's never as simple as focusing only on what the coolest web language is.