When was the last atomic bomb officially produced in the US? Why?

The US stopped building new nuclear weapons from scratch in 1991. The last model of bomb manufactured was the B83.

It does do maintenance on the bombs which includes refurbishing them. This has meant that at times the US has had to do things like figure out new ways to create old components that it has "forgotten" how to make.

There is a program in place that will make substantial modifications to one model of bomb, the B61, in an effort to prolong its effective life and increase its reliability and flexibility over the years.

As for why we stopped in 1991, that is around when the US nuclear testing program stopped as well (1992). If you can't test a new bomb, then you can't be sure it will work. All of the bombs in the US arsenal have to have been tested.

This is one of the technical and political controversies involved with the ever-aging US stockpile — can we be sure the bombs will work, 30 years after they were last really tested? If we modify them, can we be sure that the modified versions will work? If you replace all of the parts of an old bomb with new parts, is it a new bomb or an old one?

Note that when we say "bomb" here, we are talking just about the warheads. In principle, the warhead technology of the 1980s and 1990s is probably good enough for anything we might want in the future barring some very exotic uses of nuclear weapons (e.g. enhanced radiation devices, shaped radiation output). The part of the bombs that have been extensively upgraded, modified, re-worked are the external aspects connected with its "delivery." So they have added very sophisticated guidance systems to some of the gravity bombs, making them into "smart" bombs, for example, since the 1990s. They have also designed variants that can penetrate into the ground — aspects that pertain to the casing of the bomb, not the warhead. Are these "new bombs" are just variants? Under the military designation, they are just variants; for political and diplomatic purposes, it is not always clear.