Why do people romanize 新聞 as "Shimbun"?


Is there a reason why [新聞]{しんぶん} is often romanized as "Shimbun" instead of
the "Shinbun" its supposed to be? I ask as I've found that even brands like
毎日新聞 do this; romanizing their name as "Mainichi Shimbun".

How did this all start? And is it actually the correct romanization? Or is it
just a miss-romanization that people've just gone with?

