I think we've recycled this discussion as much as Reilly has recycled some of his materials, so I'm probably repeating something I wrote in 2008. Fitting, I suppose.
But I think what happened was he just got old. I've spoken with a few great longform writers about the idea of a shelf life for those guys and I think Reilly's a prime example. We wonder why he couldn't keep writing stories like his one on the suicidal high school football official or the Marge Schott feature, but it was probably unrealistic to think he could keep doing that.
Look at, say, Gay Talese. Probably the most famous magazine writer who ever lived. But when you think of his famous pieces you probably think of his Esquire pieces from 1960s and especially his features on Sinatra and DiMaggio. Both of those were written in 1966. Gay Talese since then has written a lot of really good stories, some really good (and some very strange) books but otherwise has practically disappeared. He just had the good sense, I suppose, to not churn out a weekly column for the New Yorker, where we'd notice he's saying the same things now about Anthony Weiner that he said about Gary Hart in 1988.
Dan Jenkins talked about getting bored at SI and not being what he once was. I think of guys like Rushin and Wolff and even Gary Smith, who produced such superb stuff for so long but eventually it wears on you, it's draining. And as one magazine writer told me, eventually it is easier to write a column. Less work, more pay. Less travel, more time with family, more tangible rewards. Even in that role we wish Reilly would stay sharper and fresher and more original, even if he's not producing classic 5,000-word pieces on Bryant Gumbel. But I don't think it's surprising that he's not.
Is this universal for all writers? Of course not. But it happens.
So what happened specifically to Reilly? A bunch of birthdays.