Yeah, true, Sign Language helps deaf people more than it does for being able to speak and oral better, but I think it has to do with a bigger problem. It may be more about 'unifying' Americans the simpler way possible.
Look at America's society, they want green card and VISA immigrants and citizens to at least learn to speak the english language if they intend to stay here. American's aren't interested in learning Vietnamese or Spanish to accommodate the new language, they want people to speak their language.
When you have a sign language, it has no real 'bridge' to the spoken english used by Americans, as it is just another language that exist and doesn't help them unless they learn it. So, the concept may have been seen as to patch up something between hearing and deaf so at least the people who have no knowledge of the language can at least understand some of it.
It's sorta like SimCom, if you use SimCom to teach ASL to a group of hearing students, it seems to be a lot easier than it is to go total voice off and teach them ASL. They would have a hard time adapting.