For the casual user, Pythonv3 is in many way backward incompatible and, for those type of users, who know python V2, and were happy with it, and whose full time job was something else - learning a new version of the language, particularly one without all the supporting libraries of pythonV2, didn't really have much return on investment.

Imagine someone who was used to doing something like this:

  import string
  list=open('foo.txt').readlines()
  list_strip=map(string.strip,list)

And then discovering this no longer works because the strip function has been removed from the string module (and moved to the string object). If all they want to do was a quick read of a file and then parse - they are not going to dive into Python3 and figure out the new way to do it - they'll just stick with pythonv2.