Hmm, I believe that Duldzin Drakpa Gyeltsen was one of the eight famous disciples of Lama Tsongkapa. In fact he was a very close heart disciple and was the first to be offered the seat after Lama Tsongkapa's passing but chose to decline as he wished to meditate and practice. As to why Nechung chose him, my simple mind would say that Nechung didn't specifically chose Duldzin per se but Duldzin were the who spoke to Nechung, who came in form of a dove and flew around while Lama Tsongkapa was teaching. In his devotion to his Guru, he had spoken out to Nechung to stop disturbing his Guru's teachings and later when everyone left, he spoke privately to Nechung and we know of the "promise". I would say that due his pure Guru Devotion and compassion, he got himself chosen. But it would be interesting to read what others think for me to be able to learn more.