Why was the close button moved to the left side in Ubuntu?

Mark Shuttleworth explains the reasoning behind this change on his blog.

The Ubuntu team wanted to gut the notification area (a.k.a. system tray) and add notification indicator menus instead. They also added window indicators which function similarly to notification indicators, but for windows. In order to add window indicators on the right side of the window bar, mirroring the notification indicator location, they needed to move the close/restore/minimize buttons to the left. The plan is that by Ubuntu 11.04 the notification area will be completely phased out.

There is an API available so applications may start using the indicators soon. If you can't stand this change, the buttons can be moved back by following tutorials online.

To switch it back follow these directions:

Press Alt+F2, then type gconf-editor and press enter. That’s for opening the configuration editor.

Once on this editor, in the item tree at the left, you have to look for this path app -> metacity -> general and you doubleclick on the field named button_layout.

Then, you have only to change the value field and put this:

menu:minimize,maximize,close

Edit: As far as "why?", see Trey's Answer.