Because you didn't change all, and Joomla still searching for real author in articles, k2 items and comments. Please next time dont delete old users but just rename they login and password and disable user - it would help you safe your time.
Second solution, for next time, delete all demo content and add yours from beginning. Than Joomla won't find delete user content.
Solution for now. very easy. In joomla create new user and in PhpMyAdmin change his ID from new one to "old" one. Than you can disable (not delete) this user.
I don't understand why you delete all users but leave old content, this demo content I don't beleave that you will use it on real page.