A few.

1. It takes time for investigators to do their work. A lot of these arrests are from video evidence and cross referencing that to identify a masked criminal takes time.

2. Coming in with the National Guard or with another extremely strong response would be spun as authoritarian. It would give legs to the "Trump is literally Hitler" people. Arguably this is the response the Democrat mayors were hoping to get. Trump avoided a political mistake by not taking the bait.

3. Law enforcement is supposed to be a local issue not a federal issue. Trump was respecting the principles of federalism.

4. Letting the cities go to hell while offering help to any who asked for it lut the blame squarely on the backs of local governments, all of whom are Democrats. It exposed their poor leadership for the entire country to see in a way thay simply can't be ignored or denied.