I saw a story linked here on FR (I don't have the link handy) that described the reasons as follows:
A) the clerk reported the robbery, but the officer that pulled Brown didn't know about it, meaning there was no immediate connection between the two incidents.
B) only after the "witness" (Brown's friend) spoke to the police about the robbery several days later did the police realize that they were dealing with the same people.
C) the store initially declined to give the police the videotape of the robbery when asked a few days ago (in what now seems to be a prescient desire to avoid community backlash). The police had to threaten legal action before they turned it over.
So we may very well have seen the videotape very soon after the police got it... which, unfortunately, was well after the riots and everything had started...
makes sense