Because otherwise it's just "we need to do something about that" handwaving and it gets nowhere. So, single out cruise ships - a straightforward way to reduce global pollution by a good degree with minimal effects on the individual.
The problem with ships is that once they enter international waters they can do whatever they want.
Which is exactly what they do, burn bunker fuel outside territorial waters and switch to cleaner fuel when they approach a country (because they are forced to).
There is simply no enforcement outside territorial waters, and if anyone tried to, they'd be immediately condemned by everyone else.
Is it possible that you are a member of a group of individuals that do not enjoy nor participate in cruises and therefore do not give anything up by calling for their ban? You can feel good about it without personally sacrificing anything and instead forcing others to make that sacrifice.
How would you feel if it were the inverse and others were calling for a ban on an activity you enjoy?
In many countries people eat young lamb or veal, some consider it acceptable and others don't.
It is very easy to call for a ban on something that you personally do not enjoy because you lose nothing. In fact a majority of the world's population likely does not enjoy cruising, just as a different majority does not enjoy snow sports, water sports, car racing, motorcycles, horse racing, skydiving, rugby, rock climbing, etc..
There is some form of ethical or safety concerns for each of those but the world would be a very boring place if we banned them all.
The problem with ships is that once they enter international waters they can do whatever they want.
Which is exactly what they do, burn bunker fuel outside territorial waters and switch to cleaner fuel when they approach a country (because they are forced to).
There is simply no enforcement outside territorial waters, and if anyone tried to, they'd be immediately condemned by everyone else.