The reason given for the cancellation is due to the placement of the next Saudi Arabia event, which is scheduled for Friday, June 7 (which had once been scheduled for May 3).
According to the report, not only was WWE not keen on running major events nine days apart, but the idea of holding an event called "Backlash" little more than a week after what is expected to be an event surrounded by controversy didn't sit well with people within WWE.
It should be noted that last year, Backlash was in fact held in Newark, NJ just nine days after Greatest Royal Rumble. However, the most heated of negativity toward WWE's events in Saudi Arabia didn't ramp up until Crown Jewel and the adjacent Jamal Khashoggi death controversy last fall.
The 2019 Backlash event was never formally announced by WWE, but it was nonetheless listed on their touring schedule, and San Diego's Pechanga Arena listed it as an upcoming event. WWE also had a website for the event, with the link now leading to an 'access denied' error message.