Cheaper to operate.Silly question in my opinion. There must be a need or role where L-15 is a much better fit than JF-17. No single aircraft in any airforce can best fit for all possible roles.
Flight hours apply to every sub system regardless of being used like radar and other equipment. They would have to frequently replace parts if they use JF17B for LIFT. JF17B is good for conversion flight after LIFT is done.
Trainer aircraft are also designed with much higher cycles compared to fighters