I guess because "juisi" is a borrowing from English. Isn't it beautiful that different languages express the same concept differently.
An interesting note is that Turkish also says "meyve suyu" for fruit juice, and you guessed right; it literally means "water of fruit" even in Turkish.
It's plural and it's juice. This program is getting frustrating. You have to second guess because you never know what they want. The literal translation or just, what it is. Do they want the plural form THIS time or not...... maji ya matunda translates into "juice" of course it's fruit juice. They didn't want "water of fruit.". Sorry, just getting frustrated. It's a good program though....