Because it is plural you.

If you get a sentence to translate into Swahili that uses you, it should theoretically let you use plural or singular unless something else in the sentence suggests they specifically want one or the other. Sadly, that is not currently the case. Hit the report button each time this comes up in a sentence and select "my answer should be accepted". If you feel inclined, also hit "other" and tell them exactly what the problem is.