Africa performed poorly during the boycotts era for obvious reasons. Then the agents came in and started bringing more runners on the road circuit. That was also the EPO era. African athletes were tremendous at championships on the track beginning in 1987-88, and the best Brits, Cram, Coe, Ovett, faded as they were getting old after 1988. The young and promising Brits did not rise as expected at that point and then Western nations, as Renato said, did a lot worse than in the 1980s with harmful training fads, low mileage, and discouragement with the impossibility of competing with huge drops in time from what seemed to be physically superior East African runners (who were doped up in a high proportion all the way up to the present times with the 300+ Kenyan doping positives after 2015 in an era when testing actually reached to Kenya and results were not always suppressed like they have been in Spain and as they were suppressed under Lamine Diack, who extorted athletes with positive tests and suppressed others.
The American successes in 2016-17 and especially those of the Ingebrigtsens showed Westerners that they were the only ones in their way, that they could anything on the track that anyone else could do, and it didn't hurt that all the doping cases, including the Russians who had mopped up so many medals through doping cases that continue to come up from 2009 on, were suggesting that the clean runners would have a shot. Now you have Western prodigies rising all over the place to compete for medals. Africans still absolutely dominate the marathon, where there is a lot of money and probably lousy doping control, and still have tremendous depth and competition, but this is a real sea change and hence involves more than a few factors (for example, the American resurgence started with the success of three high schoolers, Hall, Ritz, and Webb, in 2000-2001, which led to much greater depth on the high school scene and eventually the medals of the 2010s (Centro and Simpson in 2011, Symmonds, Centro, and I think Simpson in 2013, Centro and Coburn and Jager and numerous others in 2016-17). You had the rise of professional groups in the United States, especially the gray area NOP and BTC.