I saw a BA 747 being towed down one of the taxiways at LHR lately, and it was s-l-o-w. So that's probably one reason.
That would be because of the speed limits imposed on ground vehicles, even on taxiways (unless it's an emergency, of course), although I have seen tugs doing a bit more than their limit whilst towing, especially at night when the place is closed, and that can kinda put the willies up ya when you see a tug towing something past you at that sort of speed. Let's put it this way, if something happened they would never stop in time.