Look at the teams that are IN the division. 
Has Arizona actually ever been "good"? Or Great? Consistently? No, they haven't. 
Seattle? Come on , seriously? They had a good span with Holgrem and Jerry Jones feeding em picks. Doesn't last. 
Frisco is a serious mystery as to why they aren't better. After Salary Cap hell, they had a little success, but, nothing lasting. 
And the Rams? 

. They are another team that has sucked for decades only to get a little taste of the pie.. 
I would say it comes down to owners and management. And horrible picks and signings.