Well, how about his failure to disavow support from David Duke immediately during the election? (He initially claimed he didn't know Duke, although he was on record talking about him from years earlier.) Or how about campaigning on policies such as stop-and-frisk (which target minorities)?
It is a question of degrees. While politicians generally campaign to their base, they at least attempt to be inclusive once elected (for example, in state-of-the-union speeches.) On the other hand, Trump has repeatedly belittled people opposed to him (remember many months ago, Trump claimed that protesters were "unfair" to him.) He has worked partisan political messages into speeches where they do not belong. (For example, the boy scout speech, or the one he gave to the CIA.)
Other presidents, just didn't do that. Pretty sure Obama was able to keep his partisan messages to the campaign trail. I don't even remember Bush engaging in the same sort of politicizing that Trump does.
Ummm.... because he's the president, and its his job? And as such, its his responsibility to speak on national events. The death/injuries are tied to racism. Its Trump's job to acknowledge that fact and/or lay out a plan of action. A failure to acknowledge the underpinnings of racism that cause the events on Saturday is a failure to do his job.
I would expect anyone elected to the position of president to have a certain minimal competency.... pointing out "Nazis are bad, m'kay?" when it is relevant (i.e. right after a racist killed somebody) should be considered part of that minimal competency. Trump at that. He deserves no credit.