Without an atmosphere to hold onto the heat from the Sun, the side of the planet Mercury facing away from the sun 'cools' the planet more. As a result, the difference in temperature between the two sides is about 600 degrees (Kelvin)
Venus, on the other hand, has a very dense carbon dioxide atmosphere â 92 times denser than Earth. The atmosphere acts like a blanket, trapping the heat, and keeping the planet very hot.
Maybe Venus had too many private planes and limos spewing green house gases from climate change summits of eons ago.
I maybe wrong but I think it has something to do with the pressure differences in the planets atmospheres. Mercury has a very thin atmosphere and Venus as a very dense atmosphere with a pressure of 90 earth atmospheres. http://en.wikipedia.org/wiki/Gas_laws