I'm been thinking about this myself. Here's what I end up coming back to:
The best US runners used to do this. Rodgers would. Virgin and Salazar too. Lynn Jennings, Pat Porter. Deena certainly ran. It was considered a National Championships. It may have been THE National distance race of the year in the minds of many.
You may say it is still a US Champs. But so is the 15k in Florida and the 25k in Michigan. Those aren't absolute must-do for more recent generations any more than XC is. Rupp didn't line up for those every year. They're considered races for B-list Americans, I suspect, in the minds of many.
What changed? The last generation I remember that took it at all seriously was Hall vs Ritz, and Flanagan vs Huddle. That's a several year spread, but those were the top names in US distance in their primes. Those that weren't world class, but the national class crowd ran it just like they run the Marathon Trials. NCAA runners fattened up the field. Cassidy and friends go in OAR. Not just Bruce, who wins, I think but the next tier of up-and-comers.
Then it dropped off. Rupp's generation cared a lot about USATF Outdoor, but not about this. More recent waves of runners don't seem to be aware of it. Or maybe it's sort of on their radar, but it's no longer the definitive annual American distance race in the same sense that the Marathon Trials is The Event every 4 years.
Fortunately, there are occasional exceptions. What year was it recently when LRC compared the field from one of Deena's wins with Shelby's? I wouldn't mind reading that again, if Rojo sees this.... Scott Simmons' crowd stands out as an exception to the recent trend, too. Guys who can and literally have won at Outdoor are running this.
But let's face it. Today's best don't care about it like those years ago did. Why not? What's changed? If it never was a big deal, that would be one thing. But it was.
And now it's not.