The reason is simple. iPhoto is the Manager of the photos, Photoshop is the editor.
So you browse the photos in the Manager and then, should you wish to edit them, you send them from the Manager to the editor. This means you set Photoshop as an external editor in iPhoto.
You can set Photoshop (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in Photoshop or your Image Editor, and when you save it it's sent back to iPhoto automatically. This is the only way that edits made in another application will be displayed in iPhoto.
If you want to manage the photos with Bridge, then don't use iPhoto, and vice versa.