Why is the 16th Karmapa visualized rather than the 17th Karmapa in Karma Kagyu?
No, it is not true that it is done by the prescription of the Kagyu. I asked one of the senior students at the Dagpo Kagyu Ling center near me, who is very close to the head of that center, so he is in a position to know the answer. He told me that people can choose who they visualize during their practice, either the 17th Karmapa, Thaye Dorje, or the 16th Karmapa, Rangdrung Rigpe Dorje. What is true, is that there exists a special Guru Yoga practice text for the 16th Karmapa and not yet a formal Guru Yoga practice text for the 17th Karmapa.