If you look at the criminal justice system as having some amount of diagnostic sensitivity at each stage in the process (having both false positives and false negatives as all systems do), then our comparatively lower conviction rate means that (assuming end-of-the-day-justice is equal), we should be even more willing to waste people's time developing cases that have no merit (because more of the diagnostic sensitivity is in the trial/conviction stage). So it's not unreasonable to assume it's easier to get a wiretap in the US than it is in Japan -- another explanation is that Japan is substantially more likely to have false negatives (as implied elsewhere -- but the false negative rate for many crimes in the US is quite high, as roughly half of all murders go unsolved). Could also be that the diagnostic sensitivity is extremely bumpy in America or Japan (read: basically one or two steps / people get an extremely high share of the criminal justice discretion).