Hi - the reason for this is that Marketo pretty much treats each workspace as its own instance of Marketo - assets are not accessible across workspaces.
This is by design. Workspaces are essentially meant to limit users to their own environment. You can't share assets across workspaces, but you can share folders across workspace that contain certain asset types. See here:
http://community.marketo.com/MarketoArticle?id=kA050000000KyxDCASPlease note, this only works for some asset types, and forms is not one of them. If you cloned your form to a new folder within the workspace, you can then move or copy that form to the new workspace. It isn't then obviously the same form, just a clone of it.