'About' is a preposition. Prepositions need to be following by nouns or noun phrases. A gerund phrase is a type of noun phrase. It was about + the text = noun It was about + the text BEING badly written = gerund phrase '..that the text is badly written' is a whole new clause beginning with a conjunction (that). This cannot follow on from a preposition. If you want to continue the sentence in this way, you have to insert a noun to follow the preposition. The most useful noun here is 'the fact': It was about + the fact + that the text was badly written.