JMOR is on the right track in explaining voltage/amperage with his water analogy, but I will take it a little further to explain this particular question.
The question is why wouldn't a 9V radio battery substitute for the 6V tractor battery?
Think of the small battery as a tank the size of a pill bottle, full of water and pressurized to 9 psi. If you open a valve on the tank it will squirt out the water at 9 psi until it is all gone. Now think of a tank holding 50 gallons of water pressurized at 6 psi. If you open a valve the same size as the one on the first tank, the water will squirt water out at a lower rate than the first tank, but it will run out MUCH longer.
To simulate a starter, the opening in the valve would be as big as the whole small tank. The water would disappear instantly as the valve opened. The larger tank would last quite a while.
I hope this helps understand the principles of DC electricity.