Why do you think blood flukes would need to evolve this way?

Evolution may be defined as the process of change in the characteristic of the species over many generation. The natural selection is the driving force in the evolution process.

The blood fluke evolve according to the parasitic mode of the life. They have simple digestive system with the rudimentary organs. They have well developed  organ to attach with the host and has reduced body structure. Their eyes are highly reduced.