Why are many early gliders high-wing designs as opposed to newer designs? Many early gliders were designed with a high-wing configuration. An example of this is the K 8, but many more of the same and previous generations follow this configuration. 

Later generations (e.g., ASK 21) mostly follow a mid-wing design.

Is there a simple reason for this (including the change of material for fuselage and wings)?
It has to do with improvements in the understanding of aerodynamics and the various tradeoffs discovered over time. High wing has less visibility in turns, especially during thermaling. Low wing has better ground effect, but gives you less options when landing in a crosswind. High wing has better ground clearance, which helps in off-field landings.