Simply put, Rusev's return was delayed because you can't hinder Jinder!
Rusev was injured earlier this year and has been off TV for a little while now. However, he was initially supposed to be back already and according to the Wrestling Observer Newsletter, the original plan for for the Bulgarian Brute to get a Smackdown World Title showdown against Randy Orton at the upcoming Money In The Bank PPV.
But obviously those plans have changed since Jinder Mahal beat him to it and won the title against Orton a few weeks ago. This has caused Rusev's return to be put on hold as creative comes up with a new storyline for him before putting him back on TV.