Uber guarantees a rate of pay for drivers regardless of the type of service ordered to ensure participation in a service that's much cheaper for the customers. That gets expensive for Uber in what the company calls unmatched rides, in which a person orders an UberPool, but no one else gets into the car over the course of the trip. There's little revenue generated on those trips, and it can lead to Uber subsidizing the ride to ensure that the driver is paid the guaranteed rate, creating a loss for the company.