It's called the "!20 day rule", a defendant must be tried within 120 days of being served the criminal complaint.  That's not a lot of time for certain investigations by law enforcement, especially when that period also includes time for the Prosecutor to prepare for trial.  Many times it's better to delay charges to enable you to present a rock solid case because the US justice system is not designed to take every case to trial and therefore Prosecutors actively look for "weak" cases to either kill, or offer a very weak plea bargin on so the defendant will plead out early.