Why is African drama written in English language not in in African language?

Because there is no such thing as "African Language".

Africa is a continent that contains 54 countries and more than 2100 completely different languages. Some estimates place the number of languages at around 3000.

The most prominent languages spoken in Africa are:

Afrikaans

Amharic

Arabic

English

French

Fula

Hausa

Igbo

Oroma

Somali

Swahili

Yoruba

Zulu