Why did I not receive a mystery champion

Just to clear up any confusion, you were connected for the entire day of June 28th right? And you aren't under a ban or restriction?

Also be aware that distribution of rewards is not complete;

EDIT: July 2nd Mystery Champions are still being distributed, but it may take up to 2 weeks until the distribution has been fully completed. We will update this thread when we have more information!

http://boards.na.leagueoflegends.com/en/c/help-support/3VhOeAbf-pool-party-rewards-distribution-on-going

https://support.riotgames.com/hc/en-us/articles/201752934-Mystery-Gifting-FAQ