I guess the reason is it's off by default is because it was implemented later on, and to leave it on in a buggy state caused issues.
Not a big concern, but turning it on by default will perhaps have annoying compatibility issues for people using an old pygame.
If they forget to turn it on.
>>> import pygame.math
>>> v = pygame.math.Vector2(1,2)
>>> v.x = 3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: a sequence is expected
>>> pygame.math.disable_swizzling()
>>> v.x = 3
>>> v
<Vector2(3, 2)>
These are to do with assignment to a variable.