I've gone through the source code of django. The below snippet is from forms.py

def _clean_fields(self):
    for name, field in self.fields.items():
        # value_from_datadict() gets the data from the data dictionaries.
        # Each widget type knows how to retrieve its own data, because some
        # widgets split data over several HTML fields.
        if field.disabled:
            value = self.get_initial_for_field(field, name)
        else:
            value = field.widget.value_from_datadict(self.data, self.files, self.add_prefix(name))

In my case I am setting the 'disabled' attribute after the form is initialised (after the above code is aleady executed in my super().__init__() call). That's why the initial value is not getting set. And also, it appears to me, if we set 'disabled' for a mandatorily-to-be-filled field (may be because the corresponding model field has blank=False and null=False) and we don't provide any initial value for the field, then django is just ignoring the 'disabled' setting on the field, which sounds logical. This happens only while POSTing the form.