Can you explain why it takes one year after death to be baptized in the temple?
The simple and most direct answer is… Because the LDS Church has created a policy to that effect.
The Church has many policies that they use to guide the Church and its members along the right path. These policies can come into place to try to combat errors or misunderstandings. Over time, as things change the Church can change, adjust or remove these policies as the circumstances dictate.
The origin of the one year wait after death is an interesting one. During the last part of President Joseph F. Smith’s leadership and the first few years of President Grant’s leadership, a trend developed among the members. This trend took the form of urgent requests to do temple work, and to do it quickly enough so that the person who passed on could be buried in temple robes.
The church leaders found this to be a disturbing trend. They did not want the members to ascribe to the temple robes some kind of “magical” qualities bestowed on the one who passed on, nor did they want the temple ordinances to be seen as a simple check box on the path to salvation. Neither did they want to encourage the idea that some might neglect to get themselves to the temple their entire life and then have all done right at death.
So the Church leaders created the policy of waiting a year after death before any proxy temple work can be done. This policy seems to have accomplished these goals to various degrees. The Church also can grant exceptions to the policy or remove it entirely at a future date. Exceptions might be granted in the cases of people who were actively preparing and seeking the temple but didn’t make it, because they died before it could happen.