I was thinking along similar lines. This is a campaign event after all and the goal is to influence as many potential voters to vote for Trump/Pence as possible. Haley has a track record (was elected Gov. in a southern purple state) and as you say adds a diversity element. And, not every potential Trump voter shares the same hatred toward her that’s expressed here.
I don’t know why Candace Owens was not on the list, there could be a reason we just don’t know about. But there’s a clear effort to avoid having former Presidential candidates (Ran Paul being the except, but no Ben Carson, no Ted Cruz) and pundits, which is what Candace basically is. Plus with Charlie Kirk on the first night, she still might be so associated with TP USA that it wasn’t deemed to be a good idea.