I haven't seen the timing (if they started at one place and moved their way north en masse or hit everything at once), but reading some of the different stores that were hit, they're not all close together. For example, where I think that video you linked was shot is several miles from the Best Buy & Apple stores that were hit in the North/Clybourn area.

If the looters went after the various locations at the same time, that could have made it more challenging to respond to all places quickly. (I don't know if that's what happened, nor am I excusing any delays - and I hope every last person who broke the law is arrested...but it's possible they weren't intentionally letting the looters get away with it.)