Hmm... This is a very thought provoking thread. Where do I begin?
I think a lot of it has to do with the parents' attitude and bias toward speech and being able to hear just like everyone else. Sign language makes it very clear that the child isn't hearing. There are times when I think the whole point of oralism is to hide deafness as much as possible.
You'd be surprised at what some hearing say to me because they think I'm more like them then the other deaf because of my speech and I tried very hard to like them as much as possible when i was younger.
They'll say things like my thoughts must be more advanced than X because I have good speech. When I mentioned that some deaf got placed in classes for the MR when they were in the to my mother, she said to me that clearly they learned so little in school so that they deserve to be there. If you have an attitude like that you'll not get very far with the Deaf community even if you don't say so explicitly. Others will pick up on your attitude sooner or later and they won't be so friendly to you. Worse, your relationship with your deaf child will suffer as a result. No one likes to be treated like a second class citizen.