为什么Django表单选择字段不显示所选模型的初始值

from django import forms
from .models import MyModel

class MyForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)
        self.fields['my_choice_field'].initial = 'desired_initial_value'

form = MyForm(instance=my_model_instance)

if request.method == 'POST':
    form = MyForm(request.POST, instance=my_model_instance)
    if form.is_valid():
        form.save()
else:
    form = MyForm(instance=my_model_instance)

class MyForm(forms.ModelForm):
    MY_CHOICES = (
        ('1', 'Option 1'),
        ('2', 'Option 2'),
    )

    my_choice_field = forms.ChoiceField(choices=MY_CHOICES)

    class Meta:
        model = MyModel
        fields = '__all__'

<form method="post">
    {% csrf_token %}
    {{ form.as_p }}
    <button type="submit">Submit</button>
</form>