Почему я не могу задать класс полям email, password1, password2, но полю username могу

from django import forms
from django.contrib.auth.forms import UserCreationForm
from django.contrib.auth.models import User


class RegistrForm(UserCreationForm):
    email = forms.EmailField(max_length=254, help_text='Это поле обязательно к заполнению')

    class Meta:
        model = User

        fields = ('username', 'email', 'password1', 'password2')

        widgets = {
            'email': forms.EmailInput(attrs={'class': 'email'}),
        }