according to the fruitrian theory the nutrients in tree born fruit are fully developed and ready for human consumption, while the nutrients in vegetables are not. vegetable take a few weeks to grow and their minerals are "too large" for us to abosrb. while trees take years to bear fruit and therefore the minerals and nutrients are ready and bioavailalbe. that is the theory anyway.
also most fruitarians eat anything born from a tree ie - fruit, nuts, avocados, olives olive oil, dates etc.