Complex sentence: The idea for pre-orders came when people found it hard to get popular electronics in stores due to their popularity, companies were given the idea to allow people to reserve their own personal copy, which has been a huge success.
Split into simpler sentences: The idea for pre-orders came when people found it hard to get popular items in stores due to their popularity. Companies were then given the idea to allow people to reserve their own personal copy, before the release, which has been a huge success.