Probably because of our high pH soils for which there is little remedy except supplementing the plant with a product containing chelated iron.
It may also be that the tree has roots that were encircling the pot when it was planted and if it is now 8 or more years old may be larger and choking the trunk. There is really no remedy for this as removing such roots is not practical and may be too late to help.
Hopefully the tree just needs some iron applied to the roots.
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