Well,, first those big ships with 8kn speed (the official max speed in the canal) the tug boats would be useless. second, on the side of them there would be no place for a tug and total useless and at risk to be pushed into the banks by the ship, damaging the ship as it goes over the tug.(Oilspills ?) The same is for a tug in front of them. A tug aft would work as the tug
could slow the ship down that it would at least run aground with less speed. BUT,,, who pays ? First how much extra tugs the canal would need,, 100 ?,, then who pays for them,,the Canal Authority would charge the ships,,the charterers of the ships would tag it onto the average container price. and so on.
third, Who is then in charge, responsible when a ship anyhow runs aground and lets say runs the tug over and sinks it. Whoms fault it is,,the tug pulled wrong ? the ship steered wrong ? the WHO is responsible question expands greatly if it happens. From my point of view nobody involved wants that.