Because the passengers believed that they might miss the replacement plane. It doesn't take long in (error fixed) Jeddah airport before concerns about ... things not being done properly ... start to set in. Fortunately for me, because even though a Saudia employee told me that my flight would leave in the afternoon, I had lost faith and went and checked. Admittedly, she did come and find me later to correct that, but I'd already found out by then.

Because of Brexit, no compensation for late flight or compensation for additional costs once in the UK. So, even if the airline would have helped them if they had missed the morning flight, there quite easily could have been additional consequences for passengers stuck in Saudi Arabia for an extra day.