Why is Evolution the default mail/calendar package?

Solution 1:

Evolution indeed is part of the GNOME Project. That's why it's Ubuntu's default mail client, I suppose.

Obviously, this makes it better integrated into the GNOME desktop, especially when running Ubuntu. Please not that some of the following features can be achieved with Thunderbird, too, but it does require some work and doesn't come like that out of the box.

1. Notifications: Evolution makes use of native notify-osd-notifications whereas thunderbird brings its own notifications and doesn't blend into the desktop as much as Evolution does
2. Messaging-Menu: Evolution is integrated into Ubuntu's Messaging Menue
3. GnuPG-Integration: While Enigmail is good and can be found in the repos, Evolution does come with GnuPG integration without having to install any additional packages

Additionally, some features like Exchange integration are present in Evolution, too. Those, too, could be added to Thunderbird via Extensions. For private desktop use, though, Evolution provides a more integrated experience.

Solution 2:

Evolution like many other applications is part of the GNOME Project. You're free to replace it at no risk by any email-client you like better.