Happens all the time. If the cruiselines can't get enough interest in an excursion to make it worth the opperators time to show up it's dropped. If it happens on a regular basis, the excursion is dropped all together from all sailings. This is one of the reasons I never book excursions in advance although I'm in the minority with that thinking. Some don't cancel until you're onboard and then you have to stand in line with everybody else to get refunded or pick something else. Don't ever trust the excursion brochure materials they send out. They have to be printed so far in advance they're frequently useless. Looking online is at least updated info but onboard is where you can ask questions, look at new options that may have been added and get real time info.
I know there are many who will tell stories of missing something because it was sold out but that's not as common as the stories and the cruiselines would like you to believe. It does happen but with most excursion options they simply add another bus and re-open the excursions that are popular when the demand is there. Also they sometimes discount the excursions at the last minute to fill the bus.
If there is something do or die for your trip, book it early. For the rest see how you feel about it the day prior to arriving at the port and after you've seen the weather report on your cabin TV and decide then if it's something you want to do. Chances are it'll be available and possibly reduced in cost. I've even approached the opperator at the pier handling an excursion and asked on the spot if he had room for another. Now you can negotiate the cost and leave out the cruiseline if he says yes.
My way is not right for everyone. If it'll bother you to hear "nope we're full", don't do it my way.