Selection for less efficient egg transfer in Human Botfly life cycle
In multi-host complex life cycles (CLCs), an intermediate host often aids in the dispersal of offspring. 

In general terms, an animal, say a botfly, have an ultimate "purpose" or 'goal'  to spread their genetic information as much as possible. This could mean producing a large number of offspring and hoping some survive or it could mean having one offspring, investing a lot of energy and ensuring it does survive. Regardless, spreading genetic information often requires offspring to disperse (or be spread) a great distance, which may allow the individuals' genetic information to dominate an new region. 

In the case of botflies, having an intermediate host means the genetic information can be spread a great distance.

If you're looking for a more thorough explanation of complex life cycles, this paper describes it well. There is also this site more specific to the botfly which is not peer reviewed, but well-researched and descriptive.