A few (OK, maybe 10-15) years ago, new cameras weren't as common as they are now, mainly because they didn't need to be. Advances in AF modules were relatively uncommon as were motordrive advances and TBH there weren't many more areas that could be improved. In those days, sensor technology also moved very slowly, in fact, some of the sensors available back then remain unchanged to this day, although the introduction of a smaller sensor did result in smaller photosites on some larger sensors. These days however, Moore's law (among others) means that technology advances at a far greater rate, so upgrades can happen far more often. It doesn't mean we HAVE to upgrade every time a new model's introduced but it does mean that the upgrades are often a leap forward rather than a step.