Why can planes generally take more positive g's than negative g's? Most commercial airliners are rated to take +2.5g and -1.0g.  Why is this?  Is it designed that way on purpose?  To clarify, this would mean that the cost difference between -1g and -2.5 g is minuscule, so price (and thus engineering difficulty, which drives price) isn't really a factor.  

Or, are there in fact significant difficulties with creating a frame that can handle a lot of negative Gs?  (And if there are, what are they?)
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.