The ability to deploy US nukes has been useless since the end of the cold war, and now that it's starting to get relevant again, it's not reliable anymore.
And how is Germany supposed to integrate French nukes on the F-35 if Scholz wants to pursue “strategic independence” now?
Also, it was likely not a matter of cost, as the US has nothing to gain from allowing Germany to use a European plane to carry their nukes in the first place when they can sell them US planes instead.