Why is it not recommend to put V brake levers on cantilever brakes?
It's the old lever law: You can either increase force or movement distance at the expense of the other; the product force times distance remains the same.

The V-brake levers pull more cable, so they pull it with less force. If you pull cantilever brakes with a V-brake lever, that force reduction means that you get much less braking power when you put the same force into your grip.

Whether that's a problem depends entirely on you. I have strong hands, and I use good brake pads, so I can easily apply too much force on my V-brake levers, and consequently wouldn't hesitate to use it on a cantilever brake. If you have weaker hands, and/or worse pads, you may not be able to comfortably get enough braking power out of a V-brake-lever-actuated cantilever brake.