It is just because people wanted to ski differently. When people skied 70mm skis in pow, the only type of pow skiing that existed was slow, deep, bouncing S turns. That is all people could do, but it is what people liked, and what they wanted.
Then people became bored. They wanted to be able to ride the mountain with bigger, faster, more open style. It was very difficult to do this with skinny skis, so they made them larger to make it possible.
I prefer larger skis because I like to ski freely and fast.