Another answer to your question is because most of the tuning parts are made Germany as opposed to parts for a mustang. It cost more money to import it. Also BMW tuning parts are made with quality and exactness, so the person that buys it doesn't have to change anything about the part to make it fit. Lets say you buy a turbo for a 3 series, the people that made that turbo made it so that it exactly fit in the place where its supposed to not take up more more room than its supposed to.