Why does wingwalking use only biplanes? Wingwalking is an aerobatic discipline with shows such as this one.
The definition of wingwalking from Wikipedia gives no other alternative than using biplanes:


  Wing walking is the act of moving along the wings of a biplane during flight [...]


(source: Wikipedia, as of 26 February 2020)

When searching for images on the web, I can only find biplanes, mainly stearman but also other biplanes.

For me, it seems that any aircraft with high wings or parasol wings can do the job, so why do they only use biplanes?
It doesn't only use biplanes, searching "wingwalking monoplane" provides these examples, of some low wing monoplane and high wing monoplane.
Whatever the aircraft, it has to be able to fly slowly.


(source) (not 100% about this one but it looks like one Klemm aircraft)


(source)
(Here one quite rare danish SAI KZ III equipped with a hatch behind the wing, allowing acrobat Palle Johnson to get out.)


(source)
(Jaromir Wagner, Czech wingwalker holding speed record wingwalking at 270km/h here on a Britten-Norman Islander aircraft)