This post sounds like flamebait, but I'll address it anyway. The changes to an aircraft warrant a new model number when the changes are of a significant nature. "ER
" designations come when boeing (or any other manufacturer) adds extra range/fuel capacity to an existing airframe, without major structural changes. These would be changes that do not alter the overall look of the aircraft very much.
-200, -300, -400 etc desginations come with major structural changes, such as a fuselage stretch or shrink. These would be "visible" changes, as the shape of the aircraft is altered.
Then of course you have your IGW/HGW desgination...where powerplants/wing surfaces etc are altered so as to provide an aircraft with extra payload/range capabilities. Sometimes these are clearly visible changes, sometimes, you have to look harder.
In essence, if the aircraft only receives a new desgination when significant changes are made to it. It has nothing to do with having the most models in the market....it is a response to market demand. Some airlines require a long range 737 for long thin routes....others need a plane with the same capacity, but less range, to cover shorter hops without hurting the bottom line. Boeing does what it can to respond to market demands...but because the two 737s in question here are built for different puposes, both need a distinguishable designation so that the airlines know which to order for their needs.