why not是后面接的省略to的不定式，而why don't you却是在句子层面上的疑问与否定。意思差不多，参考下面两个回答
http://zhidao.baidu.com/link?url=wiAxvoJXdYXVj0Oz81qkPRHay-zb11LOdelL_mq09vq7CNldI2epDRtrrJb3Bn9mUMZ80rT4Bj7va-F9pi2yCa
http://www.zybang.com/question/2a3b77c534ab5f803eb18fbfd1ae6021.html
why not doing sth.
why don't do sth.
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