Please explain to me why it was choosen,
was it due to ubuntu copying Mac OS X or some other factor(s)?
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Mainly to save on space since it targeted towards netbooks. Read more about the rationale here: http://www.markshuttleworth.com/archives/383
To help take advantage of the limited vertical space netbooks (and widescreen resolutions) have they are integrating the menu bar into the top panel.
Also with Mac OS X you have to bring focus to an application (usually by clicking it) to switch the menu to that application, but with Unity you will be running applications full screen so there is no confusion as to what application's menu you are accessing.