Because it was never was supposed to happen with the 737NG. At one time, Boeing toyed with the idea of re-nosing the 737-300 with a 757 nose, but due to massive cost and recertification issues, the design was scrapped. Southwest had nothing to do with this decision.

Interestingly enough, the 757 was designed to have a 727 nose, but went with the current design to match production design of the 767, as well as increase cockpit space and reduce noise.