Do you really think that Trijang Rinpoche or Pabongkha Rinpoche needs training or something like that in order to grow? they can grow if they really want to, or if they want to keep a low profile it is really up to them. One thing for sure, they can definitely build a huge Dharma center than can benefit infinite amounts of people if they want to, and if that is their wish, but for now they would like to keep down.
From their past deeds and their past lives, it is pretty impossible for them to not benefit people unless they choose to stay low themselves. I am sure many, many, many people in the US know of Trijang Rinpoche's great achievements in his past lives and also that he is the root Guru of many of his Gurus. It is pretty much impossible for him to stay as low profile as he is now unless he willed it that way.
I have heard that there are assassins that is going after Trijang Rinpoche's life. I don't think that they will be successful because Dorje Shugden does protect him, and also that he does not have the karma to die that way. However it will cause a lot of negative karma on the side of the assassins. Thus from that aspect, Trijang Rinpoche is kind enough to avoid them so that they do not create harm for themselves.
Lastly, but not least, perhaps HHDL does not want him to study in the monasteries so that he is safe from negative influences or perhaps, from jealous people who would want to ruin him but he cannot say this too directly. Based on Dalai Lama's respect for Trijang Rinpoche, the words were meant to be heard for certain people, and not to Trijang Rinpoche...