I can't say if this is the actual reason it was chosen, but a reason why purple is a good choice is that, other than red, it has the lowest relative luminocity of any hue. 

So, a purple will tend to appear darker than an equivalent blue. Assuming the background is white and the text is black, and assuming we expect people to be more interested in pages they haven't already read, visited links will stand out less from a block of black text than unvisited links, but will still be noticeable.

Red isn't an ideal choice because in many cultures, it's associated with warnings and alarm.

Purple's low relative luminosity is consistent across the two most common forms of colour blindness (which are shown in this chart from wikipedia which shows them, somewhat ironically, with red and green lines... standard vision is barely visible behind red (deuteranopic, to green's protanopic) on the right in yellow):



Did they decide by studying a chart of relative luminosity? Probably not. But the fact purples appear dark could be the reason, for example a discussion could easily have gone like this:


  "We need a colour for visited links. It should look darker so they
  stand out less."
  
  "Dark blue?"
  
  "No, people might not be able to tell if a link was dark or light blue if it was the only link on a page."
  
  "How about purple? Purple is, you know, dark, it's not a bright colour like yellow or green."
  
  "Yeah, looks good to me."


Why do read links turn purple?
I purchased a new Amanda Furnace in Oct'2013. I am told in order to maximize a warm, consistent room temperature I should set the thermostat to the Fan position rather than the Auto position.  Does this make a difference in maintaining room temperature and is it more costly?
