LE officers ask Judges to seal their search warrants to prevent disclosure of the document that's public record. That alone is why we don't actually know why it's sealed but we can speculate why LE made the request.
To begin with, I think what we're going to see is a lot more of these types of affidavits and search warrants that are requested sealed just because so many more phones are being searched. A warrant is now required to request location data on a suspect from their phone carrier. Officials used to just get a court order.
As this is a pending investigation, keeping private the identities of those being investigated and others who might be helping police in the investigation would be another valid reason to seal the warrant. Police don't want us to know who they are talking to.
Another possibility here is if the suspect (e.g., alleged abductor) is at large, police wouldn't want them to know that they are on to them. Police need to look and see who is contacting who, and who is involved-- that could mean multiple phones to look at, cell phone tower dumps, and warrants required to determine their phone locations.
Also, LE made the request for residents to preserve their surveillance video. We don't know whose video has been collected or will be collected and the police probably want to keep that information private also.
MOO