because if i recall correctly, Venus has something like Super "Global warming" like imagine what we have, but multiply it by like 1000
venus does have an atmosphere, a very thick atmosphere that helps absorb, and also RETAIN heat, so it stays on the planet longer, then all that heat builds up, causing pressure, which in turn causes more heat ect.
mercury does not have an atmosphere, thus, true, all the solar radiation gets to the surface, but when that side isnt facing the sun, all that radiation that it just recieved is ejected back out into space, because there is no atmosphere to hold it in