Why do you have to wait a year before you can put the name of a deceased person through the temple?
I imagine that it’s for the same reason that people must wait a year after being baptized before going to the temple. When we arrive in the spirit world, I would imagine that there will be some acclimatizing. We’ll be living in a new environment under conditions that may be markedly different from what we have been accustomed to. There, those who have not known the gospel must have it taught to them, and those who understand the gospel but who have lived unworthily must have both the time and the inclination to repent. In the record of the great vision of the spirit world revealed to President Joseph F. Smith we read the following:
And the chosen messengers went forth to declare the acceptable day of the Lord and proclaim liberty to the captives who were bound, even unto all who would repent of their sins and receive the gospel.
Thus was the gospel preached to those who had died in their sins, without a knowledge of the truth, or in transgression, having rejected the prophets.
These were taught faith in God, repentance from sin, vicarious baptism for the remission of sins, the gift of the Holy Ghost by the laying on of hands,
And all other principles of the gospel that were necessary for them to know in order to qualify themselves that they might be judged according to men in the flesh, but live according to God in the spirit (D&C 138:31-34).
Undoubtedly that process continues for those who have entered and will yet enter the sprit world since the time of President Smith.