The roots of some trees are larger and more massive than their above ground portions (especially young trees). Usually when a tree is transplanted, it will spend a considerable period re-establishing its roots. The demands of life of the above ground portions slows this process.

Pecans and Walnuts are also difficult to transplant. I used to do a lot of grafting of pecans and walnuts and in cases where one wants a tree in a location where there isn't one ... it is better to push a whole pecan or walnut into the ground with the heel of one's boot than to move a small tree to the location. By the time a transplant recovers and begins to grow again it won't catch nor be as healthy as the one that grew from seed. And that is if the transplant survives. So a lot people I know have told me they could they could have had the pecan tree much sooner if they had known that having tried unsuccessfully to establish by transplant.

The smaller the tree the better. It will help greatly if you can irrigate and also to mulch to reduce competition.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers