I agree with epcotrulz, I suspect there have been fervent discussions with the Floridian gov't over the weekend and ongoing. No matter how badly the gov't folks want to enforce their "vaccine passport" rule, they simply can't in this case, or they'll force the cruise lines to all not run at all. So the gov't needs a way to exempt the cruise lines from the rule and presumably try to spin it in the news to save face. However you feel about it politically, this whole process would be far simpler for the cruise lines if it weren't for that rule. I'm sure Disney would rather avoid a legal battle if at all possible, so I suspect that if they thought that they could talk the gov't into a cruise ship exemption, that would be preferable to joining Norwegian's lawsuit.
I am somewhat surprised that we haven't seen the other lines joining Norwegian's lawsuit since they've all now publicly said that they're going to do what the rule specifically says that they can't. I'm sure there's behind-the-scenes wrangling going on by all of them!