none said:

Lightsabers have been shown in several spots in the movies to be able to cut through anything.

So why does Luke go all the way up to the belly of the Snow walker when he could have just as easily cut through a thin point on the walker foot, the curved ankle hinge or lateral strut: (or whatever they're called)

Luke had several seconds while the foot was planted before it began the next step.

(insert blow up Yavin IV response)

Well, as you will later see in his abysmal dealings with Jabba the Hutt, Luke has some kind of inferiority complex that forces him to kill as many people as humanly possible. I mean, sure, some Imperial forces might die if the AT-AT tips over due to a damaged leg, but if you really want to rack up a good kill count, blowing up the cockpit with an explosive is the safer way to go!