I've only ridden this ferry twice when it had maybe 300 people aboard. I think initial reports of the WTC attack mentioned 30,000 occupants. One tenth were killed. One in ten deaths of lets say 1000 passengers is no small event, especially if you know one of those 100. But 100 deaths and a downed ferry does not rock the free world, trigger and economic depression, and cause 30 billion direct damages to a community.
This is a wild guess, but I suspect that with the recourses required to assault a ferry, overpowering security and assure its destruction, a dozen car bombs could be set off at sports stadiums, parades or crowded cafes. Also, it's an all or nothing event. One thing goes wrong, and it's a US coup. I think there are better targets.
and they are?