Various reasons, for example, to prevent flight/escape (indictees may ir may not know about the indictments), to protect witnesses or to buy time while investigating other people complicit in, or involved with, the crime.

Nice overview here: https://legalbeagle.com/6790439-sealed-indictment.html

Investigations / Ops still ongoing ? 1 indictment can contain or yield up to 99 others. Mix in RICO and that fishing net can get real broad real fast. Maybe a lawfag will chime in later to explain more.