Why do some TV broadcasts show a person translating it to sign language instead of using subtitles?

I'm profound deaf. I sign, write and speak. :)

Well, sign language aren't superior. Having both : subtitles (hard hearing people) and sign language (deaf people) is better. I prefer subtitle because it is closer to the speech and i'm not fond of sign video. Often the sign interpreter is small and sign very quickly.

In general, i prefer text, it help me focusing on the content instead of the person and use less bandwidth...

Sign language still lacks lot vocabulary. It's a young language «created» in the 18s when Abée de l'Épée founded the first deaf school. And i had to create lot technical signs with sign language interpreters during my agricultural course. Furthermore, they don't have an official sign writing yet, and it is a problem for keeping human knowledge and culture outside video and technological device. So there is still lot things to do and improve.

In France, lot deaf people aren't fluent with French writing due to the lack of bilingual school (French writing and French sign language) and interpreters (eg : only 200 hours in sign language for 1 year in universities).

So, having sign language improves a lot the accessibility for deaf people as they are not fluent with writing language. For me, i prefer both. Both are good and it meet each people need. :)