There are a lot of factors that may be reasons why parents do not learn sign language. 1). They've been told that using sign language will hinder the child from learning oral language (manyof us in the teaching field do not agree with this) 2). They lack the resources of learning (no money for formal classes or babysitters used in order to go to classes, no internet to learn from websites, do not have English as native language so difficult to find books in their native languages, etc.) 3) Using sign language may mean they have to address and accept the fact the child is deaf - many parents go through years of denial 4) stigma of using sign language means (to them) that the child is not "normal" 5) Fear that the child will pick up signing faster than them (most children will) and they (parents) will not be able to catch up/ fear of being left out 6) Learning how to sign is difficult, especially because it is visual ; many parents say it is too hard to remember everything they've learned because they don't use it with other adults, just with their child. 7) They do not know other d/Deaf adults or do not interact with the deaf community
* My father never learned sign language - I think it embarrassed him. Even to his day he is in denial that I'm Deaf. Ironically he is fluent in 4 other languages (other than English). My mother started to but stopped when a professional told her that she was doing harm to my language and speech development. My mother now regrets it.
You explained it so well! This is why I belive the american society have an attitude problem with ASL. A good step in the right direction, was the pepsi advertisement, I hope the trend we are seeing now with ASL in the universities and the field of linqustics will continue, making it less frightening for parents to use ASL.