It is clear that most people learn just enough of anything to get on to the next thing they want done. The Pareto Principal says that 80% of the desired effects come from 20% of the actual work. Most people figure out which work is important toward their goal and brush past at lot of the niceties. This is true of work, languages, cultures, laws, and everything else. The plain truth is that there's a lot more to learn than we have time to learn it.
Deaf people need to know ASL. Who teaches them? Hearing people. Who taught the hearing people? Mostly other hearing people. How much did they learn? Most of the time, they learned enough but not a lot.
There are an estimated 7 Billion people in the world. It is possible that 0.1% of those people are profoundly deaf, and possibly 1% to 3% know someone who is Deaf and learned sign language to communicate with them. Not all deaf people use ASL to communicate. Not all Deaf people and their hearing companions are fluent signers. They probably know at least enough to get by within their circumstances.
If there were 3% of the people who know at least some sign language, that would mean 97% don't know any. And for that 97%, none is as much sign language as they need to get by every day. If most people do not need any sign language on a daily basis, is there any reason for them to learn anyway?
The answer is yes, but since when does reasonableness motivate people? Sign Language is not pantomime, and therefore takes years of study to become proficient. Deaf people, who can have other problems than just deafness, do not have the benefit of seeing their language in everyday use outside of families and institutions. If everyone used some sign language for at least 15 minutes a day, then society would be much more capable of including the Deaf. Communication is the greatest gift anyone can give someone else.