The family likely chose it. Note that an autopsy has been conducted, but report hasn't been released yet (as per a related CNN article). The city officials are likely choosing hold it so that the impeachment trial will conclude before it is released. The family (also per the CNN report) has said there were "medical issues", too. My view is that the autopsy report will vindicate protesters, which is why the officials are not releasing it. An FOIA lawsuit will will take weeks, and even the family would be required to file one, if the city refused to release the report.
Sicknick was a Trump supporter. The family right off said they did not want his death politicized. They also said the cause of death was unknown.
However, it seems they were persuaded into becoming political pawns. I’m sure its not hard to manipulate people on grief.
Just like they're not releasing anything on the coward who murdered Ashli Babbit.
The only people "slain" on January 6, 2021 were slain by the government.