There is, iirc, also a time limit between when an investigation begins and one an indictment is filed. The idea being to prevent the government from simply having someone under investigation in perpetuity. I suppose the ADAs COULD try and time the indictment against unrelated pop-culture events, but in my experience it seems so unlikely as to be mostly the stuff of movies. Delays more frequently where due to problems getting financial records or untangling complex webs of them, problems locating willing witnesses, problems with witness intimidation, etc..