Because it's not natural.
What people don't take into account though, is that fake-tanning and spray tans are just as bad.
People should be happy in their own skin and with what they've got.
However, people tend to be more opposed to skin bleaching because it seems more extreme. It involves, basically, burning your skin with acid. That sounds much more painful than 'painting' your skin orange.
The best way to keep healthy and happy is to wear suncream when in the hot sun, and keep your skin clean, exfoliated and hydrated with soap, body scrub and some kind of moisturiser or body butter (preferrably one that contains shea or cocoa).
Trust me, you can't go wrong with that.
Trying to change your skin colour always makes it look patchy or streaky and irregular anyway. Do you want your face to look a different colour to your body?