3 reasons:
- The really obvious one; biplanes had the interplane structural network to hang on to when doing the wing walking. It's a heck of a lot safer for the wing walker. Even today, most of the wing walker acts use Stearman or Waco biplanes.
- They generally have lower wing loadings than comparable monoplanes so could fly slower.
- They were readily available as WWI surplus in the 20s when the whole Air Circus business was getting underway. Equivalent monoplanes would be new production and much more expensive to acquire.
So while there are/were monoplane wing walker acts, they weren't true "wing walkers". The original wing walkers walked out from the fuselage to the wing tip, and the usual trick on the Jenny was to get down and hang from the wing tip protector hoop that extends down below the interplane struts. It was only possible on a biplane because there was structure and wires to hang on to.
The monoplane acts are great stunt acts, but they aren't true wing walking acts because no, you know, walking along the wing, goes on. They have a brace to let someone stand up above the fuselage, but you won't see any of them walking out along the wing to the tip without any kind of aids, as you do on a biplane.