JS is actually a decent language, and imho with more recent advancements better still. Combined with npm there is simply anything you might need available (for the most part). Not that there aren't a lot of gems out there.
In the end, having one language to think about, rather than how to shoehorn communications and logic between two languages is simply more difficult. I went, similarly from C# to node, not because I didn't like C#, but because the friction in writing JS on the server and the client was so reduced from two differing languages throughout the day.
Similarly a lot of devs went to simpler data stores so they don't have to deal with layers of ORMs that only half work, and can simply store and query basic data forms. Mongo fits this development model well, early hiccups in the technology aside.
In the end, it allows you to build web apps in one language, and not have to write translations to get from one to the other on the client.
