The 6th only covers situations where you explicitly object to a delay in your speedy trial right. Often it's in the interest of the defense to delay a trial as a tactic in preventing a conviction (e.g. by introducing a defense that the victim doesn't recall exactly). Also stupid shit like the prosecution withholding or delaying evidence that would tend to exonerate the defendant happens. What tends to happen is that the prosecution and the defendants lawyer agree to continuance after continuance. (Source: been there done that, had an open and shut self defense case that took about a year to go to trial)