Why were german immigrants referred to as the pennsylvania dutch?

The Pennsylvania Dutch are a cultural group formed by early German-speaking immigrants to Pennsylvania and their descendants. The word "Dutch" does not refer to the Dutch people or their descendants, but to German people whose ethnonym in their own language is Deitsch or Deutsch. Most emigrated to the U.S. from Germany or Switzerland in the 17th and 18th century. Over time, the various dialects spoken by these immigrants fused into a unique dialect of German known as Pennsylvania German or Pennsylvania "Dutch". At one time, more than one-third of Pennsylvania's population spoke this language.