I think another reason they are so popular is because DAZ haz a boat load of content to support their figures which cannot be matched yet
if someone create a better "mousetrap" unless they can match DAZ type support they are not going to go far no matter how good it is. users would rather use a less then perfect figure if you can get whatever content either free or for pay they may need, then to use a perfect figure with little to no content
with DAZ figures you can get just about anything you need to produce an image where as with other figures (Clark, Gloria, Open source for example) you would have to adjust to fit a 3rd party content not made for the figure, which in some cases do not work as well, or make it yourself, which most either cannot or do not do. 
however Apollo is the exception to the rule because this figure started with enough content that if you can texture or manipulate trans maps you can diversify what he came with

You might as well PAY attention, because you can't afford FREE speech