Why can planes generally take more positive g's than negative g's?
It's partly pragmatic (you start from +1.0 G, so why not optimize for that???) and partly because (most) aircraft are designed to carry human beings.  Humans as well as a lot of other things are FAR more tolerant of positive G-forces than they are negative G-forces.

There's very little technical reason that you couldn't design an aircraft wing and other structures to handle high negative G loads.  But adding that strength also means adding weight, complexity, and cost.  And since the benefit of being able to pull high negative G loads is pretty small, very few designers choose to do this.