there was no slump; that myth is manufactured and actually a fairly big lie.
demographics:
you had a baby boom in the US and those kids grew up competed up through the early 90's...and those baby boomers had some offspring who also have a peak. we're seeing the second boom. 350 million people now in the US now versus 175 million or so when Pre graduated from HS. I am counting immigrant population here, of course, documented and otherwise all of which adds greatly to our talent pool.
faster runners tackling longer distances:
up until the 2000's it was quite rare to see a sub 3:56 miler in the 10000m for the US or anywhere else for that matter. Rupp and Solinsky as ~3:50 milers have a distinct advantage over a 4-4:02 miler in the 10000m, roughly a 2sec/lap at minimum advantage and this primarily due to speed-side, not stamina per se. Solinsky was on Lagat's heels for a few seasons before he applied that speed development to the 10000m.
Bekele runs less miles than Viren but at their best, Bekele laps Viren quite easily. The explanation is found speed-side.
The first US runner to break 28:00 in 10000m would have had a hard time running 12.3sec for 100m with a running start. Rupp can run 11sec for 100m with a running start.
Goucher was successful because he was faster than the guys who ran distances for the US in the 90's -- but tough as he was he was never fast enough at shorter distances to approach 27 flat for 10000m just as those stars from the 90's were never going to go much sub 27:20 whatever the training volume, whatever the willingness to hurt.