The reason is that Raniere’s pre-sentence investigation report [PSR] has not been completed.
Such reports are assembled by federal probation officers who interview convicted defendants and others involved in their life – including victims – prior to sentencing and then send their findings to the sentencing judge.
Federal rules require sentencing to be no sooner than 35 days after the PSR is filed and the defendant has a copy of it. After that happens, there is a process whereby the defendant can make objections to the report and the presentencing officer responds.
See https://www.federalrulesofcriminalprocedure.org/title-vii/rule-32-sentencing-and-judgment/ for more details on the process.
https://frankreport.com/2020/01/02/raniere-sentencing-postponed/