For what I've heard, DAZ Studio was created because the future of Poser became uncertain at some point. DAZ was only selling Poser content at that time and in order to survive they had to have an alternative in case Poser would cease to exist. Besides, it's not DAZ that has made Poser less popular, they have offered full support to SM for their later figures, but SM chose not to support them which probably is one of the main reasons for why many have shifted to DAZ Studio.

Personally I use both, and both have their their advantages and disadvantages.