As noted earlier this week, there were late changes made to SmackDown Live when Vince McMahon ripped up the script and the show was rewritten just hours before they went on the air on the USA Network.
Aleister Black vs. Sami Zayn had not been announced yet for SummerSlam but the plan was to add it to the pay-per-view.
When the new script for SmackDown Live was rewritten, they still had one hour left to fill so the decision was made to pull Sami Zayn vs. Aleister Black off of SummerSlam and have the match take place on SmackDown Live.
Dave Meltzer discussed this on Wrestling Observer Radio: "The deal with Aleister Black and Sami Zayn was when Vince showed up and didn't like Smackdown and he tore the whole show apart, when he was done with the stuff he didn't like and [after] changing the main event they still had an hour to fill. One of the ideas for that hour was [to] put the Aleister Black match with Sami Zayn [on SmackDown]. So that was one of the things they did to fill the hole so they took that off the pay-per-view."
A Zayn vs. Black rematch is unlikely for SummerSlam but there are still discussions about adding at least one more match to the show. Yesterday, they teased adding Roman Reigns to the show in a match with Buddy Murphy.