It was a perfect storm caused by technology change. Adapterer switched and finally took to Poly after a couple years. Most of his peers went extinct pretty quickly due to several reasons:
1. Injury (Hewitt and perhaps even Oldassi and many, many more) simply could not handle the new hard hitting, grueling baseline game that developed.
2. All courters got rude awakening as passing shots started flying by them
3. Roddick adapted his game to the new reality (old forehand unreliable in the more frequent baseline barages), but was not as good. He at least survived and might have even thrived a bit in some ways as Poly seems to help serve games.
4. Wave of new players eventually showed up who were proficient baseliners and things eventually got stronger and stronger.
We can deny this as many a fraudulent fan might wish, but then denies Fed's greatest achievement; overcoming technology change. What he's doing now would be like John McEnroe being a slam threat in the year 2000. Maybe some things rolled his way early on, but he keeps adapting to extend his career. All of his peer hit a huge speed bump, while Fed put the pedal to the metal.
I'd also say the Big4/5 period quite weakish outside the top5. Normally you'd have some player that were quite good in the top 10. Roddick was around some of the time and we can say Delpo and Soderling (though I demur) would have been those players.