Upvote:9
The founders of the Pennsylvania colony (and Massachusetts, Virginia and Kentucky for that matter) were inspired by the writings of English philosophers like John Locke and Thomas Hobbes; and so emphasised that their colony's authority derived from the people by formally calling themselves a Commonwealth.
This wasn't necessarily different from other states that chose call themselves Republics (like Vermont Republic) or Provinces (like Province of New Hampshire) or Colonies (like Delaware Colony) prior to the creation of the United States.