#1. No suitable opponents
WWE's Backlash pay per view is only about a week away and while it is a very exciting time for the company, it is also a very busy one. Furthermore, between AJ Styles versus Daniel Bryan and John Morrison and The Miz versus Braun Strowman, there are very few options in the way of big-name opponents available for Matt Riddle to face right now.
Of course, WWE could always opt to pair him with talent lower down the card. Riddle could have great matches with the likes of Cesaro and Shinsuke Nakamura - but putting Riddle in this spot almost immediately pegs him as a midcard talent on SmackDown, especially if he enters into a long-term feud at that level.
At this time, rushing Riddle's debut is unnecessary and WWE would be much better off waiting until after the Backlash pay-per-view to present him properly. In all honesty, that will give him the chance to make a proper impact on the blue brand, with a number of top opponents to work with. Nobody wants to see Matt Riddle get lost in the shuffle.