There can be several reasons for this:

• The tree is not mature enough - fruit trees don’t start to give fruits until they reach a certain age.
• There are not enough nutrients in the soil or sun for the tree to absorb and use in the production of fruit.
• Alternatively, if there are too many nutrients in the soil (especially due to excesive fertilization) or the tree is pruned too much, it can spend most of its energy in growing wood (becoming taller and thicker) rather than producing flower buds.
• If the climate was too cold. If there are frosts while the tree is producing the flower buds or flowers, even if the flowers develop correctly they might not be able to set fruit.