There could be a load of reasons why he never responded. So many that it's not even worth trying to list them all. Here is an example, the piece of paper simply got blown away so he never actually got it. Another thing is our society, a guy being approached by a female sometimes means there is usually something wrong with her. Not saying all guys think this way but it can be viewed that way.
You should take this as a lesson. If there is a guy that you are potentially interested in, why not just make yourself available to him so there is no question about weather he is interested or not.
The other factor is, if the situation were reversed would you have responded? If a guy left a note on your car? Would you have called him? Probably not is my guess.