Question

Differences between past continuous and past perfect continuous .

Answer 1

Sentence construction has to match what you are trying to say, and the purpose of 'had been watching TV' is to emphasise the watching TV aspect by using a grammar construction that fronts that.

Your sentence using 'had just finished' emphasises that the TV watching has ended, and so has a different use. It is not 'much better', is is simply appropriate in different circumstances.