Why __unicode__ doesn't work but __str__ does?

it looks like you are using Python3.x and here is the relevant documentation on Str and Unicode methods

In Python 2, the object model specifies __str__() and __unicode__() methods. If these methods exist, they must return str (bytes) and unicode (text) respectively.

The print statement and the str() built-in call __str__() to determine the human-readable representation of an object. The unicode() built-in calls __unicode__() if it exists, and otherwise falls back to __str__() and decodes the result with the system encoding. Conversely, the Model base class automatically derives __str__() from __unicode__() by encoding to UTF-8.

In Python 3, there’s simply __str__(), which must return str (text).

So

On Python 3, the decorator is a no-op. On Python 2, it defines appropriate __unicode__() and __str__() methods (replacing the original __str__() method in the process).

If it's not the python 3 thing, your code as posted has incorrect indentation - not sure if copy/pasting bug or if that's how it is in the code. But your User model's methods need to be indented, like so:

from django.db import models

class User(models.Model):
    username = models.CharField(max_length=200)
    reg_date = models.DateTimeField('registry date')

    def __unicode__(self):
        return self.username