I agree. I have wondered for many years why this is so. I think because Vala was developed so specifically for use with gnome, that its maturation was mainly limited to problems in that domain. I don't think it offered anything that wasn't already mostly available between Java and C#, so it never took off for web. It also wouldn't have fit well as a GUI building language for other native platforms, so it didn't take off there. It's not a good scripting language, so it didn't take off there.
In the end, I think it lost due to the network effect.
> I don't think it offered anything that wasn't already mostly available between Java and C#
Well it is massively easier to integrate C and C++ libraries in Vala compared to C# or Java - P/Invoke is bearable, but JNI is a pain to use and very hard to get right.
In the end, I think it lost due to the network effect.