36v does not give more torq.
There is a correlation between voltage and current that determines the potential power available. IE: 36v @ 10amp is the same as 24v @ 15amp
So a 24v motor with 15amp controller will provide the same power as a 36v one at 10amp
The difference between the two comes down to the size of the battery needed in Ah terms to provide the required capacity needed to provide the required energy.
And typically its easier to configure a 36v battery that 24v.