Then along came the World Wide Web. This vaulted the application server from obscurity to the top of a pedestal, and literally scores of companies jumped in to develop products. Not surprisingly, these companies did not, and still do not, see the role of the application server in the same way. They weren't competing just to make something different. Application servers have different roles, and not every company requires the same functionality.
Scalability is a good example.
Some companies might want an application server that simply helps them organize their applications for the Web, give them better control over the business logic they contain, and make it easier to monitor and secure the data. They don't need thousands of servers. Other companies, especially big ones, do need to manage thousands of servers. For them, the scalability of an application server is crucial. So some application servers feature scalability, others feature other things, and some try to do everything.