Because I didn't know how to watch the other person(s) that I was interacting with to even notice if my communication was having the desired effect. Now there are nonverbal communication patterns that are cross cultural and there are ones that are cultural there are also ones that are idiosyncratic to each individual.
In my opinion in order to become an effective communicator one has to familiarize themselves as much as possible with patterns of nonverbal communication from these three domains.
Desmond has done a wonderful job chronicling nonverbal communication of humans across the globe and I highly recommend his work.
Check out his books on Amazon.com.