Because when you freeze something, all the water turns to ice, forms ice crystals. The longer it takes to freeze something, the bigger the ice crystals are. The ice crystal destroys the cellular structure inside the fruit, so when it's thawed, the fruit becomes mushy.
Companies who sell frozen fruits (and they can only do this well with small fruits like berries) flash freezes the produce so it doesn't turn to mush when you take it home.