Disney I think just really wants to avoid requiring vaccination in the US if at all possible.
Additionally I think Disney is being savvy "smart" in that by doing shorter cruises less chance of someone spread COVID19 on board and it being detected that way if someone was sick everyone is long gone.
Finally I think Disney also had the early data from Safe Passage. It could be possibly that a large enough % of people were going with the testing over vaccination proof that Disney finalized their plan to shift the route to one that 100% was assured not to require vaccination for cruises (since they likely have some agreement with Bahamas in place).