Let us interview an agent who will confirm that this was too much a sloped roof for an agent.
It was hot out still. The roof was hot. Police setup on near roof, kneeling and using a tripod. The shooter was laying down wearing T-shirt and shorts. It was hot. He was leaning to the R and L due to the roof heat. This explains the widening shots to the L and R as he got hot. His first 3 shots were measured and aimed. I believe the 2nd or 3rd shot is the one that grazed Trump.
Nor will we officially hear from a LEO who was told this particular rooftop was not a top priority, there were other more important concerns - notwithstanding the rooftop offered perhaps the most favorable nearby shooting position.