Probably because of the attention that came long with it. He was just in the right place at the right time for an incredibly average photo journalist. Not like the kind that go into war zones or purposefully put themselves into crazy situations. When they get that amount of attention, it's expected. Imagine only being known for a single photo that you took because you were lazy and bored enough to be at that speech instead of somewhere more interesting (without having any knowledge of what was to happen).