Imho it's all about cost
A pedalo is the simplest an cheapest thing anybody can build with a FG hull and a minimum of engineering. Anything else will cost substantially more for little improvement.
Pedals already turn in the right direction paddles
so you can make do with one single crank shaft and direct attachment of the paddels.
and you can have it sit above the water line, so you have less trouble with seals.
for a prop you need to change the mechanical direction, you have to get a propshaft through the hull, have a good seal, more complex to get it to work, more things to go wrong , break, need maintenance = higher cost. And to what advantage? it'll just need more maintenance and it will hardly go faster.