I'm going to guess one of the following;
1). They are hand-me-downs/whatever was there to use.
2). Simplicity
FYI - The Garmin Forerunner 35 and 45 are the two best watches Garmin makes. Simple, no BS you don't need and small.
If you've read anything about African Elites, especially Kenyans, then you see that they train a lot by feel and are not big on the western data analytics thing. It's their coaches that care and keep them on track in the camps. But the majority is done on pace or time to distance.
Elite Africans have K to minute break downs memorized. 2,5,10,15,20,30,40, etc...so they can look at a simple digital watch and know if their run is on pace or not.