The E. Africans have shifted a lot of their top distance runners to the roads as the prize money and appearance fees are better. And they are sending them straight to race marathons without making them establish themselves on the track.
Also, the E. African managers, agents, coaches, etc. are fairly corrupt and have done a terrible job of selecting their teams for WCs and Olympics.
But above all else, the West has finally caught up. The E. Africans broke a lot of records and had a lot of runners rattling off times that were better than US and EU records. That meant that US and EU runners had to knock down a lot of records just to start to be competitive again. From Rupp to Ingebrigtsen, those records have been shattered and that removed a big psychological barrier. Now, top kids in HS are setting their sights on sub 14 5k and no longer think that is an impossible barrier. I also think that the consolidation of high schools into huge 2500 to 4500 students also had raised the level of distance running. My son carpools with a kid who is on XC at his school. The coach said that 20 years ago, anyone who could break 18 would make varsity and sub 16 would usually make state. The school has almost doubled in size since then. Now, all the varsity boys run 16:XX and you have to break 15 to make state. It makes a big difference when a kid is coming up and they see the top kids running sub 15s. They believe that they can do the same or better.