Because choices, freedom... that kind of stuff. And I say this as a now long time KDE user who used to use GNOME in the beginning.

That being said, what made me flee to KDE was realizing that its devs somehow think they know their users better than their users know themselves so they decided to develop a software metaphor with a utterly specific way to do things. Not that it's a bad thing per se, they can do whatever they want if they don't hurt anything or anyone else - but I wish people coming into the Linux and FOSS world could have that as a kind of warning when the distro they choose to begin their journey happens to ship GNOME as default.