Because the motive and the modus operandi make a good fit for the scenario.
The family is not interested in the emotional baggage of foul play, or the hassle of an arduous investigation, which would, hypothetically, probably be fruitless, even if suspicions were accurate. It's much, much easier for loved ones to swallow an official story of a tragic accident.
For total strangers, not burdened by emotions, the scenario is objectively disturbing.