Not really sure why. I think for one the rhizome is also photosynthetic so if it dies you've only got leaves left which will also die. Additionally for ferns the leaves are simply reproductive organs, the rhizome is the real plant. I also think it's because they draw nutrients from the water column more than the substrate. They are epiphytic plants meaning they live attached to woods and rock in areas of higher flow where the only nutrients is in the water.