Because it was supposed to show how the state of things was "happy" before. Bayek is shown as a noble family man dressed in white in the flashback. We see the Pharaoh and the people celebrating, and then he looking at smiling Bayek with a suspicious look. Then we see the former clean man all bloodied, bearded and looking for revenge, in a dark lit environment which serves as a contrast to the colorful sunny landscape presented at first . It was supposed to create intrigue and make you wonder "what happened?" Something that, as you said, is told an hour later, which is fine because videogames are long.Also having a scene in which after 1 minute you get a black screen that says 1 year later is just sloppy. Why even have that scene prior to the time skip in the first place?
Can't see the problem really.