What's up with (not) tipping?

I work as a tour guide at the destination of a NCL cruise; on "ship days" the majority of our guests are coming from the cruise ship. Over time I've learned to expect to earn only about a third of the tips I receive on days with no cruise ship.

Sometimes discussing tips can feel taboo, but I figured this sub might be able to help satisfy my curiosity. Has anyone in other (tipping) parts of the world found this to be a trend? Why, in your opinion, might ship passengers tend to tip less than flying ones? What am I missing?

Edit:

Thanks a ton for your insights! My co-workers and I have been wondering about this for ever, I'm pumped to get some perspective from the ships. I'm getting a few main ideas here:

• Passengers may assume the option to tip doesn't apply to them as gratuity is included for most of their vacation.
• Passengers may be experiencing "tip fatigue" especially as our harbor is the last they visit.
• Passengers booking independent tours (like the one I work for) may be more price sensitive than other passengers and especially flying tourists. Cruising and tours can feel too expensive already.

Next ship day at the end of our 3hr tour I'm going to try to let people know, with a polite lil comment or something idk, that I'm not compensated by the ship. Maybe people don't know... I could let u know how that goes here. As far as tip fatigue/price sensitivity goes, oh well, I get it. Nothing to do about that but keep giving tours!