The 737 has been around for a long time and Boeing has a number of large customers who want 737s, the newest and shiniest 737s with more efficient engines, better fuel consumption, more passenger capacity but basically 737-shaped flying machines. South West Airlines has 750 of them, for example and they will only buy 737s.
737 pilots can switch between recent versions of the 737 easily with minimal retraining, the maintenance and ramp workers don't have to deal with lots of new hardware, any changes are incremental and reduce operating costs. Boeing really needed to design a son-of-737 which didn't have the airframe structural limits that the existing 737 versions suffer from. The resulting aircraft wouldn't be a 737 though meaning the customers would have a choice, wait for the new plane (797?) to start coming off the production line or buy planes like the Airbus 320neo now. That was something Boeing wanted to avoid at all costs hence the software-patch-for-an-aerodynamic-problem solution they eventually rolled out.