I think that's the whole problem right there, they don't have too. With absolutely zero build to pay per views, and name matches being given away every week on Raw and Smackdown, there is no mystery, there is no what if. They've seen it all.
Furthermore, when we were kids we got to fantasize about if Hogan met Flair, or if the Road Warriors met Demolition. Now all of the "name" wrestlers are in the WWE, or have been in the WWE recently, and there are no dream matches left.
I know some people may point to homegrown TNA talents like AJ Styles or the Machine Guns, or even a guy who came out of ROH like Samoa Joe as someone who hasn't worked with most of the WWE roster, but I think only a small percentage of the non-IWC fan base even knows what TNA is.
I work at a high school and when I hear wrestling being spoken of, 99% of the time it is WWE. So when WWE doesn't consider them competition, and when the average fan doesn't know they exist, how can they fantasize about Wrestler X fighting Wrestler Y in a cross promotional battle.
Unfortunately, I think it all leads back to lack of competition creatively killing the business.