Biplanes have lower minimum power and material strength demands. However these slow planes have worse glide ratios and thus distance-based fuel consumption. Today with high performance engines and materials these planes are very niche and monoplanes are the norm.
But these traits seem best for maximizing human-powered flight time. Humans are very weak engines, and the aircraft must have airliner-sized wingspans with an under 100kg weight. This presents an extreme material demand which would be lessened in a biplane design. One could argue that time in the air is more important than speed or distance for the purposes of "having a fun toy to play with". Given these demands, why are human powered biplanes not more common?