Sharp needed someone to invest in its screen manufacturing process as it risked going under. Apple helped them out by investing in new tools and production lines for new screen tech, provided Sharp produce displays for them. Sharp signed the on line.
That said, Samsung's only customer is itself these days, according to the manager of its displays subsidiary, so maybe Apple will be okay with Sharp buying back some of its production lines.
Hopefully this time their screen will be better! My iphone 5s' screen shows irregular backlighting at the top. Apple replaced the whole phone under warranty still the same! Every iphone has different screen quality. Different color temperature, some are brighter, some show light leaking some show warmer top part. Too bad I bought the iphone solely for the quality after having a cheap-looking-and-feeling galaxy s3.