The reason has nothing to do with Covid. It's a longstanding issue with most of the greedy cruise lines squeezing the dive operators so hard that the operators cannot make money (and because the operators make more money booking on their own). The operator is promised dozens of divers each week and that he is going to make a fortune. When it doesn't happen, the operator bails. The cruise lines then tell the cruise patrons that it is an "insurance issue." That is what the minions at the cruise ship excursion desks are trained to say.
This has been ongoing for years. I've spoken to cruise line management which has informed me that the puppeteers who are pulling the strings won't back down. As an aside, frequently, the dive trip cancellation for "insurance issues" happens the morning of the cruise, when only one or two divers have signed up -- and not enough incentive or revenue exists for the operator to pick up only one or two divers. Simply, it's greed, and it's not going away anytime soon.
On a more positive note, there are a few operators who won't back out for only one or two cruise ship divers and are very reliable. Anthony's Key in Roatan is one. But, be prepared not to have any complimentary "surface interval fruits" and the like, as NCL's excursion details have stated to the contrary for years. The brothers who run the Roatan show won't provide them!
In Cozumel, The Dive Shop Cozumel used to get all of NCL's business -- until the shop disbanded. I spoke to Ulises, the "Gerente Cozumel" (manager) a while ago about what happened and if I recall, NCL squeezed that dive shop too hard as well. Anyway, here is a great tip: if you want to dive from a cruise ship in Cozumel and something falls through, as a last resort, show up at Marina Fonatur (particularly if you are on Carnival or another line that uses that port that is very close) and solicit a seat on any available boat that can get you back to the marina in time -- but like others have said, most boats leave earlier than you will get there. However, some shops take their boats out later, like at 09:30, and if it is a bigger boat -- not a "fast boat," there are decent odds that you'll find space on it. This is not a preferred method, but if you're in a pinch and get dumped by another operator, and don't have any contacts in Cozumel, that is the last-minute methodology I would use. Problematically, the operator won't be expecting you, so there may be some last-minute adjustments for your equipment and, if no suitable extras onboard exist, someone may be driving back fast to their shop to get whatever you are missing.