Back before NodeJS was cool and was still just a struggling new project, I listened to a podcast interview with the original author. The original developer wanted to create something that was equivalent to Twisted (a Python framework) but was able to use the V8 JIT compiler from Chrome. He thought that Twisted was fantastic and was a real fan of it, but Python didn't have a working JIT (Pypy didn't work with Twisted at the time) and he thought he could build something faster.
The justifications along the lines of "use the same language for front end and back end" came a lot later on and seem to be very much after the fact. The original reasons were that Twisted was really cool and V8 was really cool and sticking the two together somehow would be even more cooler. This sort of enthusiasm is how stuff that actually works tends to get built.