The short answer is: because Lotus Notes goes beyond traditional e-mail...
I came across this FAQ from the University of Windsor's IT team and I think they've done an outstanding job of describing a few of the reasons that Lotus Notes is more than e-mail:
You probably already know that one of the components of Lotus Notes application takes care of receiving and sending e-mail messages and other documents. But Lotus Notes is more than just another e-mail software. Behind the Notes interface that you see before you is a sophisticated system for storing, managing and transmitting information, backed up by a team of well-informed support people located right here on campus. Notes empowers groups of users by offering them a secure delivery and storage system for exchanging documents and ideas in a fast and convenient manner.
You will soon realize that Lotus Notes Mail is equipped with features that exceed that of any software package of its class used on campus to date. At first, you might be using Notes only for e-mail, but soon you will discover the true power of Notes that will allow you to perform different tasks in a quick and convenient manner . There are many advantages in store for those who choose to unleash the full potential of Lotus Notes. With Notes you can store, sort, organize, respond, spell check, embed, view and attach; all with the simple click of a button. Notes combines all the most advanced features of other e-mail applications. It integrates your e-mail with calendar that is built into your mailbox, address book, "ToDo" list and the Web. It streamlines daily information management. It offers workflow and collaboration applications, such as TeamRooms, Document Libraries, Discussion Forums. Notes security options insure that shared and private documents will not go public. Room & Resource Reservations application allows you to book rooms and resources, such as data projectors, right from your calendar when you schedule a new meeting. And there is more to come...
These folks appear to get the value of Notes and I'm pleased to see they are making efforts to communicate those benefits to others. They have a large number of Notes tech notes and FAQS that you may find helpful, too.
Source: University of Windsor FAQ 24