One possibility is that it was the BURGLARY investigation unit that was first sent out on the case. I believe the assumption must have been that someone did a BREAK and ENTER on the VD property to kidnap Danielle.
Since the VD's insisted Danielle MUST have been in bed asleep (even though neither had actually seen her since 1030pm at the latest, possibly even earlier).
Since the VD's insisted Danielle MUST have been in bed asleep (even though neither had actually seen her since 1030pm at the latest, possibly even earlier), then police felt they were dealing with a potential robbery that went bad. (My guess, anyway).
The question about why Robbery unit and not homicide, was brought up several times in these threads, but that was long ago. Some others might have more info than I have on this.