Because the native language of the deaf is sign language. Spoken language has very little in common with sign language. Text is a written form of a spoken language for those who hear so it isn't too hard to learn. For someone who can't hear it is completely different from anything.

Imagine the time where you knew something of a second language, but were not fluent yet (some of you may have never tried, but most people have done at least a little of a foreign language), watching a program in that language with subtitles in that language. That is what text subtitles are for a lot of deaf.