All these answers are good but for the real answer: because reddit did a thing where they had "X"porn around like, idk, '07 ish. Earthporn, spaceporn, unixporn, etc. It's just (supposed to be anyway) very pretty pictures of pretty things related to the topic, like for instance a scenic waterfall for earthporn, some cool nebula with a bunch of colors added in for spaceporn, and a sick i3 rice for unixporn.