Forgive me for rehashing such a hackneyed idiom, but I can explain why Python makes sense for web development in one statement:

data = {"foo": "bar"}

Executing the above in Python, followed by: print(data["foo"])
results in the output:  "bar"

This would be mind numbingly straightforward except for the following exercise, which should be able to be completed by even more readers than the above, because they don't have to have Python installed.

Copy the statement: data = {"foo": "bar"}

In between script tags within the head tags of an HTML document, then set the onload attribute of the body tag to: alert(data["foo"])

This too results in the output: "bar"

Furthermore, not only do Javascript and Python share the same general syntax for defining the above literal representation of an associative array -- Javascript "objects" and Python "dictionaries", but they also share the same syntax for defining literal ordinal arrays:

arr = ["foo", "bar"]

Now, in either language, accessing: arr[0]
results in: "foo"

It gets better.  These structures can be arbitrarily nested within one another.  JSON, or "Javascript Object Notation", starts with an associative array as its outermost structure, but there's no reason to not start with an ordinal array:

superbowls = [{}, {"winner": "Green Bay", "loser": "Kansas City", "score": "35-10"}]

After defining the above in either language, the statement: superbowls[1]['winner']
results in the same output in both: "Green Bay"

Javascript and Python do not necessarily allow all types of data to represented identically, so in order to take advantage of their commonalities subsets of each would be required.  As a for instance, in Javascript booleans can be represented by the barewords true and false, whereas in Python the same values would be represented by the capitalized versions True and False.  Both languages however evaluate 0 as false and non-zero integers as true, so this case is easy enough to work around.  It won't always be so easy, but still it seems promising and worthwhile.  One carefully constructed Javascript/Python data structure could be used to configure both client side and server side functionality, debunking the myth that the two tiers must be engineered in two entirely different manners, thereby doubling development costs, etcetera.