They address this briefly in the PEP[0], and it's largely because they want to be very clear about when it's happening. The distance between

    if x = y:

and

    if x == y:

is very small, and easy to ignore, so insisting on

    if x := y:

makes it very clear that what's happening won't be mistaken for comparison at a quick glance.

[0]: https://www.python.org/dev/peps/pep-0572/#why-not-just-turn-...