Why does python disallow usage of hyphens within function and variable names?

Because hyphen is used as the subtraction operator. Imagine that you could have an is-even function, and then you had code like this:

my_var = is-even(another_var)

Is is-even(another_var) a call to the function is-even, or is it subtracting the result of the function even from a variable named is?

Lisp dialects don't have this problem, since they use prefix notation. For example, there's clear difference between

(is-even 4)

and

(- is (even 4))

in Lisps.

Because Python uses infix notation to represent calculations and a hyphen and a minus has the exact same ascii code. You can have ambiguous cases such as:

a-b = 10
a = 1
b = 1

c = a-b

What is the answer? 0 or 10?

Because it would make the parser even more complicated. It would be confusing too for the programmers.

Consider def is-even(num): : now, if is is a global variable, what happens?

Also note that the - is the subtraction operator in Python, hence would further complicate parsing.