This question should be in the history somewhere as I recall a fairly long discussion with a lot of points, not just the main ones that ExXB so helpfully listed.
This one might be a similar thread;
it talks about an APU powered motorized nosewheel
Is Delta still using this?
Also, apparently, the strain on the landing gear - which was not designed for fully laden towing - causes issues which offset any savings.
Is it possible to tug but not via the landing gear? Is there a possibility to have some sort of attachment points or pulling lugs? I guess that's for the designers of the future though.