WW1 was the 1st air war, he was the top scorer, he died in combat, the press lionized him, he had a nickname that caught on in the media and with the public, his memory was not overshadowed by anything else, since he died in 1918. Those are the main reasons.
People remember pioneers, and he was one of the pioneers of air combat, and one of the most memorable, he was, "famous".
Not really a difficult question.
Why do people know who Wayne Gretzky is?
...Btw, the French ace was Rene Foncke, whos official score was 75, although I am not sure how that breaks down into confirmed as opposed to claimed.
As for Richtofen, his career and life in WW1 is very interesting stuff, well worth reading if you can find a decent book about him.