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, at that point in the films it really hadn't been established that they could cut through anything. But I have an explanation that I like for my own personal version of Star Wars:
While lightsabers ARE capable of cutting through many things, it isn't necessarily that easy to do. Much like how there are historical accounts of samurai who could cut through multiple (as many as 4 or 5) human bodies (through the mid section, spine and all) with one strike. That doesn't mean anyone who picks up a katana can do that. You must be extremely skilled and have a tremendous amount of practice with proper cutting techniques. In other words, just because the sword is capable doesn't mean the person weilding it is. So I like to think lightsabers are much the same as a katana. The sword itself is capable of incredible things, but only if the person weilding it has extreme amounts of training and skill.
And to get back to your original question... Since, by that battle, Luke had recieved relatively little training in his short time with Ben, he wasn't capable of cutting through giant pieces of metal, even if it was possible.