Sprint's been playing catchup as they build out capacity and speed on their 4G LTE network. They've tried to make up for it by offering some really good deals to sign up new customers, which makes them an attractive offer for bargain-hunting mobile users looking for the best possible deals.
There's a big glimmer of hope from the iPhone 6 - it's the first iPhone to support Sprint's "Spark" service, which offers faster data speeds on their network. It's something already available on some Android phones, but the iPhone 6 will be Apple's first phone to support Spark.