Assassinations are surprise killings of prominent individuals for political purposes. Targeted gang killings are an interesting case, because they are political within the context of intra- and inter-gang politics, but not viewed in that light in a broader context. If I was watching a documentary about two rival gangs, I probably wouldn't blink twice at someone referring to a hit on a rival leader as an assassination. In every day conversation, it would probably be weird, because the normal assumption is of the broader political sphere.

People are calling this an assassination because they are making the (probably reasonable) guess that the reason to shoot Charlie Kirk during a political speech is to make a political statement.