The other factor is the NATO nuclear sharing arrangement. The F-35A is the only new aircraft certified to carry the US nuclear weapons under that arrangement, so that impacts Belgium, Germany, Italy, and Netherlands. Germany looked into certifying the Eurofighter Typhoon for the nuclear strike mission but decided that they couldn't afford it, and bought the F-35A instead. Of course, if the US pulls back from NATO and ends nuclear sharing then that concern would become moot and some of those countries would be likely to develop their own nuclear weapons.