Because if you live somewhere where electricity is cheap, then mining is profitable. Mining bitcoins in an area where electricity is expensive makes about as much sense as growing bananas in the desert, and in that case you're better of buying your bitcoins (and bananas) from someone who can produce them cheaper than you can. People wanting something that they can't produce as cheaply or as easily as someone else is pretty much the whole reason trade exists.
What is considered 'cheap' electricity?
