I believe that would be because taps on mobile devices trigger pointer events instead of mouse events (which kinda makes sense since there is no mouse).

So if the PJS source code is checking for theses events

document.getElementById('canvas').addEventListener('mousedown', function(){mouseIsPressed = true;});
document.getElementById('canvas').addEventListener('mouseup', function(){mouseIsPressed = false;});

then nothing is going to happen on mobile because the mouse events are not triggered.

For it to work how you want you would have to check for pointer events like this

document.getElementById('canvas').addEventListener('pointerdown', function(){mouseIsPressed = true;});
document.getElementById('canvas').addEventListener('pointerup', function(){mouseIsPressed = false;});

I do not have any mobile devices to test it on, but I think that adding this code to any PJS program will make it work on mobile

function scopeOut (p) {
    return function(){return this;}()[p];
}
var outputCanvas = scopeOut('document').getElementById('output-canvas');
outputCanvas.addEventListener('pointerdown', function(){mouseIsPressed = true;});
outputCanvas.addEventListener('pointerup', function(){mouseIsPressed = false;});

I have the same problem with the canvas webAPI, which makes sense