They know that he will have a top of the line, hardball defense team. So LE and the DA have to have everything in perfect shape before his arrest.
This case is more complicated than it looks, imo. There were a lot of people that came and went from that busy house. Many probably knew codes and/or had keys because people had to come take care of house situation during tour dates.
The keys to the Tesla were probably in the home too. And if Celeste was not on the tour, then those same 'helpers' would have access to Celeste too.
D4's defense will probably point to some of those people---Personal assistants, agents, house sitters, drivers, photographers, session players, etc----so many people become enmeshed with a singer/influencer's daily life. The singer relies upon these managers/agents/stylists/session players/trainers/personal assistants for their every day life activities.
So the defense is probably going to be able to say that D4 left for his tour, and persons X, Y and Z all had access to his home, his car and his friend Celeste. That will make things murky if any of those people were in and around that house that week.
Also, we may not ever have a verified cause of death. Maybe she OD'd or harmed herself and so someone wanted to hide her body? I doubt it but the defense can take it there.