Expanding on your comment, it's analogous to torque vs speed in cars. You wouldn't use a Ferrari to tow just because it has a high top speed and lots of top-end power. You'd use something with high torque down low and good gearing.
Similarly, tugs use different propeller setups (e.g. ducted props for extra power, or vectored systems such as azimuth thrusters or cyclorotors for extra manuverability), which just cause extra drag if all you're doing is cruising at a fixed speed throughout oceans.
A big container ship trying to tow something is not unlike having a high-speed racecar spin its wheels. It's not designed to put that torque down at low speed and lift a big trailing mass.
Similarly, tugs use different propeller setups (e.g. ducted props for extra power, or vectored systems such as azimuth thrusters or cyclorotors for extra manuverability), which just cause extra drag if all you're doing is cruising at a fixed speed throughout oceans.
A big container ship trying to tow something is not unlike having a high-speed racecar spin its wheels. It's not designed to put that torque down at low speed and lift a big trailing mass.