For example there is a lot of text editors, media players, text editors, mail clients, etc, but only one of each group is selected to be delivered as pre-installed in the next version of Ubuntu. So, I'd like to know how are those applications rated to become a pre-installed application?
Do it need to be not that great so that applications other developers have written have the chance to be installed by the user? For example, let's say that the application AAA is the best of its group, but if this application is used as default/pre-installed, other applications from the same group will probably not be installed by the users because they will know that AAA is the best.
Default applications in Ubuntu (except the phone) are decided by the Ubuntu Desktop Team https://wiki.ubuntu.com/DesktopTeam .
Their mission is to produce a desktop that is:
Their goal for the default applications on the install image is to provide a great experience for a wide variety of users, everything from business to gaming to internet surfing to multimedia. They must also keep the image size as small as possible for faster downloading. This means that the install image must be broad but shallow - which generates complaints from some users that the image is too sparse in their preferred specialty uses.
They do not like to change applications (which would violate "Consistent") without good reasons and plenty of discussion on the mailing list.
Ubuntu used to inherit a lot of its default apps from Gnome, which was the default desktop.
That gave us Nautilus the file manager, gedit the text editor, gnome-terminmal, Totem the movie player and a lot of other default apps. Earlier on it also gave us e.g. the email client Evolution and Epiphany for web browsing.
Ubuntu also inherits from Debian, and that gives us defaults like Firefox the webbrowser (Iceweasel in Debian).
Other apps have simply been selected for being "best of brand" and contributing to the best desktop experience, like Brasero for creating CDs and Banshee for playing music (except Banshee is no longer default AFAIK, Rhythmbox is back). I don't think "best of brand" are DEslected, I just think the default app for an action must be one of the best available for that purpose, and must fit in seamlessly in the desktop experience.
I suspect the last reason is why the arguably better VLC is not the default video player - it's not as integrated in Gnome and Unity and drags in more dependencies.