I don't think she wanted to go before Trump. She would have talked about policies that Trump would take apart.
Not sure of when Trump is scheduled to visit, but if she goes after Trump she can try to score points by dissing his statements, without making any points of her own. She has to wait to see what Trump says so the policy wonks can do up a scripted press/photo op for her.
Have the feeling she isn't smart enough to go toe-to-toe with Trump ad lib, and she has that reflex, inappropriate cackle. She'll be taken apart unless tightly scripted.
Harris can't risk another public relations disaster at this point.