Single-engine taxi is used for taxiing out (to the runway) and taxiing in (after landing) when the aircraft/operator allows it.
It is usually done when the taxi time is big, which means at big and busy airports. Use Google Earth to see how complicated big airports are in case you're not familiar with the airport layouts. But for the purposes of your question, let's consider a simple airport.
As shown, there is a single runway and one major parallel taxiway. Typically airplanes land and takeoff in the same direction.
First off too many tow trucks is too much traffic, too much money, and an increased chance of accidents on the ground.
Let's say in the above picture the plane is towed to where it says (11) for takeoff, the tow truck will detach, then what? It will have to occupy the runway and exit (delaying departures and arrivals).
Same thing for arrivals, tow trucks will need to wait by the exits, and with the time it takes to attach, the exit will be occupied. If not the exit but farther down, then you're already almost at the gate.
So the why not is basically: more traffic that doubles when the tow truck detaches, expenses (trucks, drivers, maintenance), reduced runway capacity, bigger chance for ground accidents, and a very complicated coordination task.