Why does wingwalking use only 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)


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


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