I don’t think it’s the passport thing. The differences between European passports are minor, so in that matter you surely could accept all EU nationalities. If you really want the best ones, then Sweden, Finland, France, Italy, Austria and Switzerland are among the ones that bring you the farthest in the world, and those are not in the list, while Greece and Norway are less powerful passports, and the USA, Canada and Australia even less, and all of them are in the list.
I don’t think it’s the passport thing. The differences between European passports are minor, so in that matter you surely could accept all EU nationalities. If you really want the best ones, then Sweden, Finland, France, Italy, Austria and Switzerland are among the ones that bring you the farthest in the world, and those are not in the list, while Greece and Norway are less powerful passports, and the USA, Canada and Australia even less, and all of them are in the list.
https://www.passportindex.org/byRank.php
Of course it could be one or multiple specific countries they want you to travel, but chances for that are low. Clearance sounds much more likely.