Q: Why can't I migrate ALL my photos from iPhoto to iOS 8?

have you enabled the "Summarise Photos" option? Then disable it.

By default, Photos displays a representative subset of your photos when you view by year or by collection.

To see all your photos, go to Settings > Photos & Camera, then turn off Summarize Photos.

Also, have a look at this link:  Migrating from iPhoto for iOS to Photos on iOS 8