Because the
cherry blossoms and the
Hanami festival are so important to the Japanese, they bred the trees to have as many blossoms as possible, resulting in a truly splendid display when they bloom - you literally can't see the tree for the blossoms.
However, this breeding had a weird side effect: the trees are barren, because they have so many blossoms that if they did bear fruit, the branches couldn't support the weight and would snap. Thus, even though the country is chock full of cherry trees, cherries (like most fruit) are an expensive luxury.