Having read the book on his single college year (sorry, forgot the title!) and observing his career from afar (ie the UK!) a couple of thoughts spring to mind.
Firstly, he did not deal well with adversity in his college year as described in the book. He allowed a winter injury to ruin his outdoor season when he was clearly in good physical shape. Is he one of those guys who wa superb when everything went absolutely 100% in his build up but couldn't adapt easily to the slightest set back?
Secondly, following from that, he seemed to lack confidence. I know he is often slammed on this site for perceived arrogance but really he went into races with clear plans and then lacked the confidence in his ability to put those plans into effect.
Thirdly, against the watch he has a fantastic range - from 800 to 10K. But as a racer he was less effective, especially outside the US. Contrast his career with Steve Scott's - someone who clearly loved to RACE!
Fourthly, it does just happen! To mention two British greats; Ovett and Cram went pretty quickly from international class to relatively mediocre quite rapidly despite, apparently, still putting in the work. Perhaps the body can only take so much high level racing and training, maybe the mind can only force the body on for so long? In 1964 Snell won two Olympic golds. In 1965 he was not the same runner; ran just about OK in the US then had a pretty mediocre tour of Europe - and retired. He said physically he was not much different than the previous year but mentally he no longer had it. Did Webb just lose that deep down inside, burning desire to excel?