Why are some many node.js web frameworks building based on Express? Why Does Express do not implement those features?

I am a newcomer to node.js techique stack. I found some many articles are introducing node.js and Express. I know Express is a good web framework including template engine, routing, etc. It is popular and active framework. There are some point I am confused:

1. Why there are still so many web frameworks are built on Express? Is it lack of much features?
2. Why does Express do not evolve to add more features? Is it a lower-level web framework？
3. What are the product orientation for Express and others (which is built based on Express)?

Express is designed to only do a few things well. It will take care of routing and some templating for you, but it isn't a complete MVC solution. Since express takes care of a lot of basic middleware required for MVC frameworks, these frameworks are built on express.