Practical considerations I would assume: Django dominates the large majority of Python jobs, while Rails dominates the large majority of Ruby jobs out there. PHP has a much more divided split.
Python 19,381 jobs http://www.indeed.com/jobs?q=python&l=
Django 1,057 jobs http://www.indeed.com/jobs?q=Django&l=
Django doesn't even dominate Python web related jobs if you accept "Python JavaScript" as a proxy for Python web related jobs: http://www.indeed.com/jobs?q=Python+Javascript&l= gives you 4,671 jobs. Granted for many of them Python is not the main skill asked for.
Python is used by sysadmins and by scientists and even in financial services combined with C++. The combination of Python and C++ is requested 7,165 times. Compare that with "PHP C++" 3,051, "Ruby C++" 2,277, "Java C++" 25,675 and "C# C++" 13,045.
So Python has become a language that is very broadly used in all kinds of development jobs.
We are discussing web development jobs in this particular thread.
Django doesn't even dominate Python web related jobs
Yes, it does. SimplyHired: Django 1168, Pylons 143, TurboGears 18, Zope 96, web.py 5.
"Python JavaScript"...Granted for many of them Python is not the main skill asked for
Exactly the problem with that approach. It's relatively common to list a bunch of languages including Javascript, Ruby and Python in desired experience for any non-MS web development job, but pretty uncommon to list specific frameworks unless that's what the job is for.