Why can't you create/open multiple instances of iTunes?
iTunes uses a database file to maintain a track of all the meta data on the contents of the iTunes library.  Opening multiple instances of iTunes would potentially result in the 2nd instance being unable to gain the correct access to these files as they would be locked by the first instance.

OK, you can have multiple iTunes libraries (hold Option when starting to choose or create a new one), but this is a relatively edge case scenario, and even for users of this functionality, I cannot see many instances where it would be useful to have more than one open simultaneously.  Playing 2 files at once would sound awful, and if you want to copy from one to another, you can still gain access to the files directly through the finder.