hooks call order and hooks dependency
Your problem is actually the opposite of what you're thinking. A call to file_stream_wrapper_get_instance_by_scheme triggers hook_stream_wrappers() if it hasn't been called already.

However, the problem is that the stream wrapper hook was already called before your module was enabled and the information from that is statically cached. So you need to clear the static cache to get your own stream wrapper:

drupal_static_reset('file_get_stream_wrappers');