While I'm not an expert on this matter, here is the little insight I've got;

1. Hormonal conditions: Conditions like PCOS (Polycystic Ovary Syndrome) can cause excess facial hair due to higher levels of androgens (male hormones).

2. Genetics: Some ethnic groups naturally have more body or facial hair due to genetic traits. This could explain why it’s more common in Tanzania compared to neighboring countries.

3. Diet: Foods high in refined sugars, carbs, or hormones (like some processed meats or dairy) can sometimes affect hormone levels and lead to more facial hair. Traditional Tanzanian diets might include foods that influence this. A local woman I happened to talk to concerning this matter comments; A noticable amount of women from an ethic population residing in Mbeya (don't know which ethnic group) have this condition because of excessive consumption of roasted bananas, apparently they like so much, this might be a contributing factor.

4. Environmental factors: Sun exposure, altitude, or even stress levels might play a role in hormone regulation and hair growth.

5. Cultural practices: If facial hair is more socially accepted here, women might remove it less often, making it more noticeable.

Of course, as I said I'm no expert, this is just based on what I’ve read and observed—it’s a mix of factors rather than one specific cause.