My suspicion on why they cancelled it is due to the sharp uptick in COVID cases in Greece the past days/weeks.
https://www.worldometers.info/coronavirus/country/greece/
I am in London right now--this is my 5th/6th trip to the UK (depending on whether you count the initial shut down) since the pandemic started--and as much as it seems the world is ready to re-open, I think we are heading into a really uncertain period, and ABD should cancel the remaining of the fall European trips. This is from someone who is obviously very proactive about International travel and can't wait to get back to "normal." But the Delta variant is no joke, and the potential for serious disruption of these trips is very real. Not to mention the hoops that you have to jump through--hoops that aren't consistent and can change at any minute--are a nightmare for planning purposes.
I was in London about 6 weeks ago as well when cases were just starting to go up. Look what has happened since then with a country who has done extremely well with vaccinations. Cases have exploded. The good news is that if you are vaccinated the vaccinations seem to be lowering the curve of hospitalizations and death, but that doesn't help the portion of the population who is not vaccinated. There is also a potential for a new variant due to the "variant facotories" of the unvaccinated. My point is that in any country (US included) where you are starting to see a slight uptick ... be ready.
Organized/group travel right now is very problematic due to the fact that you are somewhat reliant on the behavior of other people around you. Right now, I feel a certain level of "control" in that most of my family is fully vaccinated (my daughter lives in England and only has her first shot as she's 21--she can get the second in a couple weeks), we can wear N95s, avoid eating inside, moderate our activities/calculate risk, and pretty much avoid other people as much as possible. This is not possible with a group trip. I also can't understate the amount of stress that is involved with testing, etc. In the UK right now, travelers from the US have to do 4 or 5 tests: pre-test to enter, day 2 test, test to release on day 5, a day 8 test, and a test to re-enter the US. Even just the last test is very stressful with 3 people. The three travelers in our party are fully vaccinated and have been since late February, but there is always the chance that one of us could test positive, and we'd be forced to isolate another 10 days (or longer if a second persons tests later). I'd be way more worried if we'd been sitting on a bus with fellow travelers who weren't wearing masks properly, who weren't vaccinated, etc, or inside tight places (i.e. many of the tourist sites).
We are signed up for an October ABD River Cruise and being on a boat/ship is a whole other can of worms. Disney seems very resistant to mandating vaccines, which is IMO the only responsible thing when you are talking about putting people on a ship in a closed environment and a potential for a disruption of travel with positive cases. I understand that you aren't eliminating the possibility with vaccinated travelers only, but you are definitely lowering the risk/chance of problems. I just googled, and in the past couple of days two cruise ships have cancelled/returned due to a few/handful of positive cases (Cunard & American Constellation). Is ABD really going to run river cruises when there is every chance that COVID cases are going to send the ship back and impact other passengers trips?
Don't get me wrong, I actually think we are going to have to adjust to COVID and start living our lives again. But International travel is different, and with the number of countries involved (and different rules), the uncertainty, and the potential for disruption and massive headaches with Delta running rampant, I don't think opening it up as we are heading into the colder months makes any sense. They also haven't issued any guidance on any of these trips, which to my mind suggests they aren't ready.
I know some of the counties these trips are going to have relatively low numbers right now, but Spain, Greece and Portugal have all opened travel and are seeing upticks. Even small upticks now, can turn into big upticks in a couple months (see England) with Delta. I think France is starting to see a slight rise, too.
I'm worried. I hope things settle down, but after being in the thick of it the past two weeks, I am very pessimistic as we head into the fall/winter.
I have become disheartened with ABD though, so I may just be cranky 
