It's a single lane tunnel and is thus one way. The parking are can only hold so many waiting vehicles and queued passengers. Their options for adjusting capacity are severely limited.

Then you consider what might happen if the lead vehicle in a convoy becomes disabled, or worse, starts on fire.

It's the same reason planes are safer per "passenger mile traveled" but aren't as safe per "total journeys taken." If you crash a plane you stand to injure or kill hundreds of passengers at once.