In universe, I think Pyrodante has it...
Out of universe? The hooded look is one most people associate with Monks, Religious zealots, wise old men in caves, Wizards and the like.. Lucas was sticking to a tropean instantly recognizable stereotype to cause people to automatically make certain assumptions, most of which are consistent with Jedi.
- Wise
- Secretive
- Celibate (until the new order)
- Possibly Powerful (even monks were, in the middle ages)
- Mysterious
- etc.